In English,
まだ is regularly translated as ‘still’, or ‘not yet’, depending on if it is used in a positive non-past, or negative sentence. The reason that Japanese only requires one word for both of these nuances is due to
まだ literally meaning ‘something is unexpectedly still going’.
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まだ走（はし）っている。
I am still running. (Although you probably didn’t expect it, I am still running)
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まだ親（おや）がいるからゲームできない。
My parents are still here, so I can't play games. (Although you probably didn’t expect it, my parents are still here)
-
まだ宿題（しゅくだい）があるから、遊（あそ）ばない。
I still have homework, so I can't hang out yet. (Although you probably didn’t expect it, I still have homework)
From these sentences, we can see that
まだ may be translated in each case as the following.
(Non-past) - Although (A) is expected to not be there/happening, it is. ‘Still’ in English.
(ている) - Although (A) is expected to not still be going, It is. ‘Not yet’ in English. The negative form of this will be discussed more in
まだ～ていません.
This is the opposite of
もう, which means ‘something is unexpectedly finished’.
まだ may also be used to ask questions. This is when the speaker thinks that something ‘might’ be continuing, but wants or needs to confirm.
まだ is often used simply as an expression indicating that something is ‘lacking’, or ‘not yet’ where it needs to be. In these cases
まだ is frequently repeated as まだまだ, for emphasis.