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What is the part of speech (word class) of to-infinitive?
as example:
I forgot to send the letter.
I: the part of speech (word class) is pronoun and the word function (clause element) is subject.
forgot: the part of speech & and the word function is verb.
to send: ??
the letter: the part of speech is noun (the is determiner) and the word function is object.
In traditional grammar, the infinitive ("to X") is a noun, the form of a verb that names the action of the verb.Daily Grammar.
I want to play the piano.
The subject is "I". The verb is "want". The direct object is "to play" the indirect object is "the piano".
Among current grammarians, however, infinitives are not thought of as nouns in English. A summary of some current thought may be found here: Education First.
For the best understanding of how such forms as infinitives might be viewed by grammarians, one will need to study from time to time what is written about them.
In your example:
I forgot to send the letter.
"to send" is the direct object of "forgot", no matter what part of speech it might be.
