还【hái】

还 goes before the verb and says that something is still going on, or that there is more of it.

还没 and 还不

还没 says something hasn't happened up to now, and leaves open that it will. 没 alone says it didn't happen; 还没 says it hasn't happened yet. The verbal 了 drops, as it does after any 没.

• × 我还没吃饭了 → 我还没吃饭

还不 goes with states and abilities – understanding, knowing, being able to – which are not events that happen once.

Adding one more thing

还 adds to what has already been said. It overlaps with 也, but carries a sense of the list growing.

With 可以, 好 or 不错, 还 means "reasonably" rather than "still": 还可以, 还好, 还不错.