「まだ」 situates the present time inside a time segment that stretches back into the past and is either expected to end or the end is being considered as a possibility
ex: 「宿題はまだしなかった。」 means “(I) still haven’t done homework.”
exists in a section of time marked by “not having yet done homework” that contrasts with a future section of “having done homework”
the present time is being contrasted with a past point in time where “should have already done homework”
ex: 「田中さんはまだ若い。」 means “Tanaka-san is still young.”
ex: 「田中さんはまだ大人ではない。」 means “Tanaka-san is not yet an adult.”
ex: 「日本語は上手いね。」「いいえまだまだです。」
example of 「まだ」 meaning “not yet” with a non-negative copula
「もう」
「もう」 situates the present time inside a time segment that stretches forward into the future and compares it to the past when current conditions were different and to the future where the transition is expected to happen
ex: 「宿題をしなさい。」「もうやったよ。」
the condition of “having done homework” has already happened in the past
there is another 「もう」 that is similar in meaning to the particle 「も」 (meaning “another”)