Formal:
* まだ終わっていません (mada owatte imasen): This is the most literal translation and is appropriate in formal situations. It means "it is not finished yet."
* まだです (mada desu): This is a shorter and more casual way to say "not yet."
Informal:
* まだだよ (mada da yo): This is a very casual way to say "not yet."
* 終わってないよ (owattenai yo): This is a more informal way to say "it's not over yet."
Example sentences:
* 会議はまだ終わっていません。 (kaigi wa mada owatte imasen.) The meeting is not over yet.
* 宿題はまだです。 (shukudai wa mada desu.) I haven't finished my homework yet.
* ご飯はまだだよ。 (gohan wa mada da yo.) Dinner isn't ready yet.
* 映画はまだ終わってないよ。 (eiga wa mada owattenai yo.) The movie isn't over yet.
The best way to choose the right expression is to consider the context of the situation and your relationship with the person you are speaking to.