The individual word "sentence" is a common noun. However, a sentence itself is comprised of many different parts of speech.
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.