This dictionary says that yathābhūta is an adjective, whereas yathābhūtaṃ is an adverb
I haven't seen how yathābhūta is used in practice. Where is yathābhūta even used? I can only find yathābhūtaṃ.
But grammatically I presume that an "adjective" would qualify a noun (e.g. "he sees a true thing"), whereas an "adverb" qualifies a verb (e.g. "he truly sees a thing").
