What part of speech is used for subjects and subject complements?
Just keep in mind, subject and verb agreement issue, subject/object ( complement position) are always a noun or pronoun.
look like adjectives? But by functions these are nouns too.
Many were present there. e.g.
Problem is phrase or phrases.
Please keep in mind, the subject of a verb and subject of a sentence does not mean a same thing.
The cost of new trucks is/are high, a valid question of it.
Here, subject of the sentence is ===the cost of new trucks
But subject of the verb is only ---cost!
A single word that is the important to determine the sub-verb agreement issue.
the cost of new trucks===is a phrase.
The same way you will see at the complement position too--
that usually disturbs many people who are unwilling to understand the math of GRAMMAR, at least 80% issues, grammar is a math!
20% are exceptions that famous grammarians say.
These 20% is trouble makers too unless ones are not articulate at basic issues that I notice every day among learners.
Like, why varsity or university in spelling? Why sincerely yours, where there is not any verb at all to modify---sincerely? Ask like these many.
Solution: Understand the grammar by morphology or like a math, then you will find, the beauty of grammar.
The cost of new trucks is high. Because cost ( a single word) is the subject not trucks. Or you will not find the subject or object ( complement) after a preposition.