He forgot to mention ...
Okay now in this part of a sentence. "He" is the subject, "forgot" is the verb but what is "to mention" ? Is it an adverbial or it is subject complement ?
Forget is a catenative verb. That is, it allows a verbal complement. "To mention" is the catenative complement of the main verb.
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Forget is a catenative verb. That is, it allows a verbal complement. "To mention" is the catenative complement of the main verb.
Of course, the sentence is incomplete. "Mention" would be followed by its own complement (a clause or direct object).