Did the Buddha copy Hinduism and add his own stuff? Why is Buddhism connected to thoughts in Hinduism if he didn't do so?
In his introduction to the translation of the early upanishads, Patrick Olivelle writes-
The Upanisads were composed at a time of great social, economic, and religious change; they document the transition from the archaic ritualism of the Veda into new religious ideas and institutions. It is in them that we note for the first time the emergence of central religious concepts of both Hinduism and of the new religious movements, such as Buddhism and Jainism, that emerged not long after the composition of the early Upanisads. Such concepts include the doctrine of re-birth, the law of karma that regulates the rebirth process, and the techniques of liberation from the cycle of rebirth, such as mental training associated with Yoga, ascetic self-denial and mortification, and the renunciation of sex, wealth, and family life. Even though theoretically the whole of the vedic corpus is accepted as revealed truth, in reality it is the Upanisads that have continued to influence the life and thought of the various religious traditions that we have come to call Hindu. Upanisads are the scriptures par excellence of Hinduism.
Some of Buddhism's cardinal doctrines like karma, reincarnation and ahimsa, compassion are found in the pre-Buddhistic early upanishads.
First, let us take the Brihadaranyaka upanishad. Regarding the date of this upanishad, the following are the opinions of most scholars -
The exact year, and even the century of the Upanishad composition is unknown. Scholars have offered different estimates ranging from 900 BCE to 600 BCE, all preceding Buddhism. Brihadaranyaka is one of the first Upanishads, along with that of Jaiminiya Upanishad and Chandogya Upanishads. The Brihadaranyaka Upanishad was in all likelihood composed in the earlier part of 1st millennium BCE, around 700 BCE, give or take a century or so, according to Patrick Olivelle. It is likely that the text was a living document and some verses were edited over a period of time before the 6th century BCE.
Thus, this upanishad was before the time of the Buddha, as per almost all scholarly opinions.
Further, there are about five pre-Buddhistic upanishads, according to scholars, and the Brihadaranyaka is one of them.
The early Upanishads all predate the Common Era, five[note 6] of them in all likelihood pre-Buddhist (6th century BCE),
[note 6] The pre-Buddhist Upanishads are: Brihadaranyaka, Chandogya, Kaushitaki, Aitareya, and Taittiriya Upanishads.
Death and transmigration have been described in the Brihadaranyaka upanishad -
I am referring to the above link only for the Sanskrit text. I will give translation of Patrick Olivelle in order to remove any possible accusations of hindu bias.
.....tena pradyotenaiṣa ātmā niṣkrāmati—cakśuṣṭo vā, mūrdhno vā, anyebhyo vā śarīradeśebhyaḥ; tamutkrāmantaṃ prāṇo'nūtkrāmati; prāṇamanūtkrāmantaṃ sarve prāṇā anūtkrāmanti; savijñāno bhavati, savijñānamevānvavakrāmati । taṃ vidyākarmaṇī samanvārabhete pūrvaprajñā ca || 2 ||
Here I am giving Patrick Olivelle's translation (not from the above link).
Then the top of his heart lights up, and with that light the self exits through the eye or the head or some other part of the body. As he is de-parting, his lifebreath (prana) departs with him. And as his lifebreath departs, all his vital functions (prana) depart with it. He then descends into a state of mere awareness and develops into one who is thus endowed with perception. Then learning and rites, as well as memory, take hold of him.
tadyathā tṛṇajalāyukā tṛṇasyāntaṃ gatvānyamākramamākramyātmānamupasaṃharati, evamevāyamātmedaṃ śarīraṃ nihatya, avidyāṃ gamayitvā, anyamākramamākramyātmānamupasaṃharati || 3 ||
tadyathā peśaskārī peśaso mātrām apādāyānyannavataraṃ kalyāṇataraṃ rūpaṃ tanute, evamevāyamātmedaṃ śarīraṃ nihatya, avidyāṃ gamayitvā, anyannavataraṃ kalyāṇataraṃ rūpaṃ kurute—pitryaṃ vā, gāndharvaṃ vā daivaṃ vā, prājāpatyaṃ vā, brāhmaṃ vā, anyeṣāṃ vā bhūtānām || 4 ||
Olivelle's translation again -
"It is like this. As a caterpillar, when it comes to the tip of a blade of grass, reaches out to a new foothold and draws itself onto it, so the self (atman), after it has knocked down this body and rendered it unconscious, reaches out to a new foothold and draws itself onto it.
"It is like this. As a weaver, after she has removed the colored yarn, weaves a different design that is newer and more attractive, so the self, after it has knocked down this body and rendered it unconscious, makes for himself a different figure that is newer and more attractive—the figure of a forefather, or of a Gandharva, or of a god, or of Prajapati, or of brahman, or else the figure of some other being.
Now, let us look at a different section of the upanishad -
tadyathānaḥ susamāhitamutsarjadyāyāt, evamevāyaṃ śārīra ātmā prājñenātmanānvārūḍha utsarjanyāti, yatraitadūrdhvocchvāsī bhavati || 35 ||
sa yatrāyamaṇimānaṃ nyeti—jarayā vopatapatā vāṇimānaṃ nigacchati—tadyathāmraṃ vodumbaraṃ vā pippalaṃ vā bandhanātpramucyate, evamevāyaṃ puruṣa ebhyo'ṅgebhyaḥ saṃpramucya punaḥ pratinyāyaṃ pratiyonyādravati prāṇāyaiva || 36 ||
Patrick Olivelle's translation -
"It is like this. As a heavily loaded cart goes along creaking, so this bodily self (atman), saddled with the self (atman) of knowledge, goes along groaning as he is breathing his last. 36 Now a man grows feeble on account of either old age or sickness. "It is like this. As a mango or a fig or a berry detaches itself from its stem, so this person frees himself from these bodily parts and rushes along the same path and through the same opening back again to a new life (prana).
The above quotes show that the doctrine of reincarnation is present in the pre-Buddhistic brihadaranyaka upanishad.
Now, let us come to the doctrine of karma in the same upanishad. (The bold highlighted Sanskrit below shows that the exact word karma was used in this upanishad. So not only is the doctrine of karma found here, but also the exact word karma was used to describe the doctrine).
yathākārī yathācārī tathā bhavati—sādhukārī sādhurbhavati, pāpakārī pāpo bhavati; puṇyaḥ puṇyena karmaṇā bhavati, pāpaḥ pāpena | atho khalvāhuḥ kāmamaya evāyaṃ puruṣa iti; sa yathākāmo bhavati tatkraturbhavati, yatkraturbhavati tatkarma kurute, yatkarma kurute tadabhisaṃpadyate || 5 ||
tadeṣa śloko bhavati | tadeva saktaḥ saha karmaṇaiti liṅgaṃ mano yatra niṣaktamasya | prāpyāntaṃ karmaṇastasya yatkiñceha karotyayam | tasmāllokātpunaraityasmai lokāya karmaṇe || iti nu kāmayamānaḥ; athākāmayamānaḥ—yo'kāmo niṣkāma āptakāma ātmakāmo na tasya prāṇā utkrāmanti, brahmaiva sanbrahmāpyeti || 6 ||
What a man turns out to be depends on how he acts and on how he conducts himself. If his actions are good, he will turn into something good. If his actions are bad, he will turn into something bad. A man turns into something good by good action and into something bad by bad action. And so people say: 'A person here consists simply of desire.' A man resolves in accordance with his desire, acts in accordance with his resolve, and turns out to be in accordance with his action.
On this point there is the following verse: A man who's attached goes with his action, to that very place to which his mind and character cling. Reaching the end of his action, of whatever he has done in this world— From that world he returns back to this world, back to action. "That is the course of a man who desires.
A different translation is given in wikipedia, but the essence is similar.
One of the earliest formulation of the Karma doctrine occurs in the Brihadaranyaka Upanishad. For example:
Now as a man is like this or like that, according as he acts and according as he behaves, so will he be; a man of good acts will become good, a man of bad acts, bad; he became pure by pure deeds, bad by bad deeds;
And here they say that a person consists of desires, and as is his desire, so is his will; and as is his will, so is his deed; and whatever deed he does, that he will reap.
— Brihadaranyaka Upanishad, Hymns 4.4.5-4.4.6
Another instance of the doctrine of karma in the same upanishad -
....tau hotkramya mantrayāṃcakrāte; tau ha yadūcatuḥ karma haiva tadūcatuḥ, atha yatpraśaśaṃsatuḥ karma haiva tat praśaśaṃsatuḥ; puṇyo vai puṇyena karmaṇā bhavati, pāpaḥ pāpeneti | tato ha jāratkārava ārtabhāga upararāma || 13 ||
Patrick Olivelle's translation -
So they left and talked about it. And what did they talk about?—they talked about nothing but action. And what did they praise?—they praised nothing but action. Yajnavalkya told him: "A man turns into something good by good action and into something bad by bad action."
It has been shown above that two of the cardinal doctrines of buddhism - karma and reincarnation - are already present in the pre-Buddhistic upanishads.
PS: This can become a long post, so I will either write a separate answer for other points or will extend this later.
Buddha used ideas common to the Indian spiritual circles of his time, including Hinduism, but was far from simply copying them. Some of these ideas he strongly opposed, like the one of eternal atman. Some of them he included into his teachings, like karma/kamma, but gave it different meaning, more in line with his stressing importance of ethical quality of our intentions and subsequently our actions.
'Hinduism' did not exist when the Buddha was alive, as follows from Wikipedia:
This "Hindu synthesis" started to develop between 500 BCE and 300 CE, after the end of the Vedic period (1500 to 500 BCE), and flourished in the medieval period, with the decline of Buddhism in India
When Buddha was alive, the religion of the Brahmin priests we call 'Brahmanism', which was far more limited in scope than Hinduism, per the stock description from the Pali suttas below:
Now at that time the brahmin...had mastered the Three Vedas, together with their vocabularies, ritual, phonology and etymology and the testament as fifth. He knew philology and grammar, and was well versed in cosmology and the marks of a great man.
The Three Vedas are explained in Wikipedia as follows:
The canonical division of the Vedas is fourfold (turīya) viz.,
Yajurveda (YV, with the main division TS vs. VS)
Of these, the first three were the principal original division, also called "trayī vidyā"; that is, "the triple science" of reciting hymns (Rigveda), performing sacrifices (Yajurveda) and chanting songs (Samaveda). The Rigveda is the oldest work, which Witzel states are probably from the period of 1900 to 1100 BCE. Witzel, also notes that it is the Vedic period itself, where incipient lists divide the Vedic texts into three (trayī) or four branches: Rig, Yajur, Sama and Atharva
If you read the Brāhmaṇa Vagga in the Majjhima Nikaya, there was no Hinduism. In fact, Hinduism, as inferred by Wikipedia (in the 1st quote above), probably copied from Buddhism.
For example (while I am happy to be corrected), I do not recall reading in the Pali suttas any detailed discussion/debate between the Brahmins & the Buddha about respective theories/doctrines of karma & reincarnation. In the little I researched, I found no systematic doctrine of karma & reincarnation in the Three Vedas. Even when I browsed some later 'Upanishads', I found no mature systematic doctrine about karma & reincarnation (apart from the ideas of one certain individual philosopher).
Again, if we read the Brāhmaṇa Vagga in the Majjhima Nikaya, we find the Brahmins were similar to the Hebrews (and probably many other tribal groups), where their belief system was about how they were 'God's chosen people' or 'born from Brahma's mouth/breath' (while the other castes were born from lesser parts of Brahma's body).
Some useful links from Sutta Central, with lots of links, books & research, are:
Note: While the Pali suttas never mention the 'Upanishads', the reincarnation orientated leader of Sutta Central, namely, Bhikkhu Sujato, appears intent to follow conventional historians rather than follow the Pali suttas, in his efforts to back-date the Upanishads to prior to Buddhism. If we place our trust in the Pali suttas, we find what existed prior to the Buddha was the Three Vedas & their accompaniments.
An answer about this topic by Bhikkhu Yuttadhammo is here: Did the Buddha ever mention the Upanishads or the Vedas?
Lastly, if you wish to be a real Buddhist, it is not possible to believe the Buddha copied any of his core teachings from Brahmanism/Hinduism because the suttas say what the Buddha taught he "never heard before". "A Buddha" cannot copy other doctrines otherwise he/she cannot be called "A Perfectly Self-Enlightened Buddha" ("Sammasambuddha").
'Arising, arising!' — At this thought, monks, there arose in me, concerning things unheard of before, vision, knowledge, understanding, light.
'Cessation, cessation!' — At this thought, monks, there arose in me, concerning things unheard of before, vision, knowledge, understanding, light."
Vision arose, insight arose, discernment arose, knowledge arose, illumination arose within me with regard to things never heard before:
Buddhism is not what you can see today. Once the real dhamma is translated and written in Sanskrit, the real meaning has disapeared. Due to this when reading dhamma today, people get a closer meaning to hindunism. (Since they know hindunisn) Which is not the case. Buddha had advised not to translate dhamma to sanskrit, but someone has done. That's how it is. So just because it seems like the same thing, it is not what budhha taught...!
Hindu,muslim,buddhism.Christian This is only different label. But truth never different.. Only one.. For example. Indian,pakistan,america,China Are different label. These believe belong to our mind but Earth there no division. Only one.
You aren't belong any country and any regions.. Mind believe create division from human beings..