1) If you use a dash, Duo will accept it anyway, it's not sensitive to punctuation.
2) We don't use a dash there. It's used instead of це or before це (not sure about this one). E.g. Коти — тварини. Коти — це тварини. (not sure if a dash is needed or це replaces the dash).
3) These = ці
Це can either be a declarative pronoun ("this milk" -> це молоко), or... a.. I don't know the name for it. A pronoun pronoun :D It's not attached to some noun, it means "this" in the sense of "it".
Це учень -> This/It is a student.
Цей учень -> This student.
Це цей учень -> This is this student (sounds weird, but let's use it for the sake of an example)
So, це and це simply overlap. "This is this milk" would be "Це це молоко" :)