Exposition
Yes, elements in a sentence are capitalized, such as proper nouns, the first word of a sentence, and titles.
The subject in the sentence "clean your room" is "you."
The predicate of this sentence is "is my friend's cousin".
You didn't write a sentence, you wrote a question. So there isn't a third word in that "sentence." Or if you are trying to be a smart aleck, "this sentence" also doesn't have three words, but in that case you didn't use correct punctuation. In either case it shows you don't really have a good grasp of the English language.
stone and cerement
It in symmetry with sentence a is what? What is a sentence with symmetry in it? This sentence with symmetry is symmetry with sentence this.
It is the first sentence of a paragraph which is the topic sentence.
Who or what the sentence is about is the subject of the sentence.
Who or what the sentence is about is the subject of the sentence.
The subject of a sentence is who or what that sentence is about.
yes
the meaning of sentence is sentence
A subject in a sentence is who, what, or where the sentence is about.
As a question, it can be a sentence, like: A sentence can have one word, and as a question then it can be a sentence: Who?
compound sentence, simple sentence, and complex sentence
a descriptive sentence is a sentence that describes something in one sentence