まだ～ていない

「まだ～ていない」 expresses the state of "not yet done, or still in the process of doing but not completed" regarding an action.
This structure is often encountered in conversations with 「もう～ましたか。」("Have you already done...?"). Accordingly, to answer the question「もう～ましたか。」, you can respond in three ways: 「はい、もう～ました。」 ("Yes, I have already done.")
「いいえ、まだです。」 ("No, not yet.")
「いいえ、まだ～ていません。」("No, I haven't done it yet.")