まだ + verb

Note that verbs that are used with まだ cannot be used in the past tense:

まだしませんでした。
should be replaced by
まだしていません。

The sentence means "I haven't done it yet", so since the state is ongoing, the progressive form has to be used.

まだ  can also be used with the present tense:

まだ寒いです。
Mada samui desu.
It's still cold.

まだ暖かくならない。
Mada atatakaku naranai.
It hasn't become warm yet.