まだ can be used to express when something is not yet the case, or is still the case.
For affirmative forms of verbs まだ means still. Example:
- Are you still studying Japanese?
For negative forms of verbs まだ means not yet. That said, it still means still when you think about it!
- I haven't been to Edinburgh yet.
- I still haven't been to Edinburgh.
まだ is often contrasted with もう. See the notes on もう.