"It" is always a pronoun, unless within quotation marks.
subject
noun
Noun, and part of an adjectival phrase.
"When" is an adverb when it begins a sentence.
I believe it would be an adjective.
An auxiliary verb.
it depends what word it is it could be any part of speech depending on the sentence
In a sentence, the word "action" can function as a noun.
In the sentence "The time is up," the word "up" functions as an adverb. It indicates the completion or expiration of time, modifying the verb "is."
In this sentence, "past" is functioning as a noun. It refers to a period of time that has already occurred.
The part of speech that answers "what" or "whom" in a sentence is a pronoun. Pronouns are words like "he," "she," "it," "they," "who," and "what" that replace nouns in a sentence.
All subjects are part of speech for they are all nouns.