还是【háishi】

还是 between two options turns the sentence into a question that asks the listener to pick one. The answer is one of the options, so there is no 吗.

• × 你喝茶还是咖啡吗？ → 你喝茶还是咖啡？

What goes on each side

The options can be nouns, verb phrases or whole clauses. Repeating the verb in the second option is common.

A verb and its negative make a blunt yes-or-no choice.

还是 in questions, 或者 in statements

或者 also means "or", but it lists options in a statement and asks for no choice. English uses "or" for both, so learners reach for 或者 in a question.

• × 你要米饭或者面条？ → 你要米饭还是面条？

A choice can also sit inside a statement after 不知道 or 问, and it still takes 还是.

还是 also means "still" and "had better"; those are separate points.