Off the top of my head, subject is the noun in the sentence that does the action. So, in the sentence, "John goes to the store", John is the subject. The subject compliment is this: "It is I who knew him best." In that sentence, I is the subject compliment. The object of the preposition is a noun that follows a preposition, and is the last part of a prepositional phrase. I explain best with examples: "I went in the house" The object of the preposition is "house". Look up a list of prepositions; there are many. A few are, "in" "on" "into" "to", et cetera. I can't see your post so I can't remember the other questions you had; I will post another one.