Yezzir,
but not a very great one
"She will never agree to that" is a complete sentence, and does not contain a possessive pronoun. If the sentence said, "Her cousin will never agree to that," then the word 'her' would be a possessive pronoun.
One is the preposition "with" -- "I agree with the proposal." But it could also be "to" -- "I agree to the proposal."
Life is hard, don't you agree?
"The armadillo is a poor swimmer." is a complete sentence
It is a complete sentence.
Complete the sentence with "He".
Using the word "then" at the beginning of a sentence is not incorrect. However, it does not make a sentence complete. A complete sentence must have, at a minimum, a subject and a verb. "Then" is neither of those.
I presume, assume, you mean, what is the meaning of a complete sentence? A complete sentence has a noun and a verb. I'm glad that I am no longer studying another language.
Yes. You would only need a period at the end of the sentence in order for it to be a complete simple sentence.
Yes, a semicolon could indeed join an incomplete sentence and a complete sentence.
Yes that is a complete sentence
No, a complete sentence needs a subject and verb at least.