A single word that is the subject of a sentence must be either a noun or a pronoun. However, some phrases and clauses, which are not considered to have parts of speech in themselves, can be the only subject in a sentence.
All subjects are part of speech for they are all nouns.
subject
The subject!
If you make '1987' the subject of your sentence it would be a noun.Normally, it's a numeral (a cardinl numeral).
the subject
In this sentence "it" is used as the subject of an impersonal verb and is therefore a pronoun.
An active verb is required; any other part of speech may be present.
In this sentence, "you" is a pronoun. It is the subject of the sentence and represents the person being addressed or spoken to directly.
A contraction. I is a noun/subject of a sentence. Am (the 'm) is a verb.
* adjective * modifies the subject of the sentence 'novel'
The phrase "I was then too young" is a complete sentence comprised of a subject ("I") and a verb ("was"), therefore it is a declarative sentence. It does not represent a single part of speech as it contains multiple elements functioning together.
In the sentence "tomorrow is a mystery," 'tomorrow' is a noun functioning as the subject of the sentence.