まだ～ていません is a regular expression that combines the 副詞（ふくし） (adverb) まだ, with the 接続助詞（せつぞくじょし） (conjunction particle)
て. The
て form will be followed by the verb
いる in its negative
いません (polite), or
いない (casual) form. This expression has the meaning of ‘to still not have done (A)’.
-
その映画（えいが）はまだ見（み）ていません。
I still haven't watched that movie. (I continue to not have seen that movie)
-
それはまだ生徒（せいと）に教（おし）えていません。
I have not taught my student that yet. (I continue to ‘not’ have taught that to my student)
From these examples, it can be seen that this grammatical structure follows the regular rules of conjugation for the
て form.