In the example sentences of my textbook for the grammar 〜まま, it is used either with 「に」 (①) or without 「に」 (②,③): In the case of ②, I understand that 「に」 is not necessary as まま is not acting as an adverb. Again, at ① it is used as an adverb so 「に」 is attached to it. What bugs me is the example sentence ③. I do not understand why 「まま」 is not attached「に 」even though it is modifying the verb:
Initially, I thought it might be one of those cases where a particle is ommitted in some instances and both possibilites (namely attaching 「に」or ommitting 「に」) are equally acceptable.
However, I encountered the same sentence later on as part of an exercise where I am asked to pick one option from either: so obviously, the only possible option is not to attach「に」to 「まま」just like in the initial example sentence ③.
Why? What am I missing here?
Your help is much appreciated.
EDIT: Thanks to user3856370 for pointing out here that the「に」in ① comes from the grammar pattern noun+になる (and similarly, noun+にする), where 「まま」 is the noun.The example sentence ① is one of such cases.