Do is always a verb either a main verb or an auxiliary verb.
The action in a sentence is the part of speech known as a verb.
regular speech
In the part of speech, the main verb is the word that expresses the action or state of being in a sentence. It is the key element that carries the meaning of the sentence.
Past is being used as a noun in that sentence.
seventeen is an adjective in this sentence
The action in a sentence is the part of speech known as a verb.
it depends what word it is it could be any part of speech depending on the sentence
regular speech
All subjects are part of speech for they are all nouns.
you
Pronoun
Noun
Him is a pronoun.
a sentence is a part of speech
Why is the beginning of a interrogative sentence.
In the sentence, "Which is his sister?", "which" is an interrogative pronoun.
Noun--however, many words have several possible parts of speech; please submit the Whole Sentence when asking for part of speech--cannot be sure without seeing how it is used in the sentence, as that's what determines the part of speech.