还是 (háishi) means "or" in questions. You put it between two choices, and the other person answers by picking one: 你喝茶还是咖啡？ "Would you like tea or coffee?" It is a key question pattern in HSK 2.
A + 还是 + B？
Put 还是 between the two things the other person can choose from. The word order stays the same as in a normal sentence; you just replace one word with "A 还是 B". The question mark is enough: the sentence is already a question.
When the choice is between two actions, say the whole verb phrase on both sides of 还是. When the choice is a time or a way of doing something, the verb usually comes after each choice: 你今天去还是明天去？
You can also put 是 before the first choice, as in 你是坐飞机去还是坐火车去？ It makes the question sound a little more careful, but it means the same.
A 还是 question is already a question, so never add 吗. 你喝茶还是咖啡吗？ is wrong; say 你喝茶还是咖啡？. You can add 呢 at the end to make the question sound softer, but not 吗.
To answer, just pick one of the choices and say it in a short sentence: 你喜欢猫还是狗？ is answered with 我喜欢狗。 Don't answer with 是 or 对.
还是 can also sit inside a statement when the statement contains a question, for example after 不知道 (don't know) or 问 (ask).
In a plain statement such as "I drink tea or coffee, either is fine", Chinese uses a different word for "or": 或者, which you will learn in HSK 3. You will also meet 还是 meaning "still", as in 他还是很忙 (he's still busy). That is a different use and is not covered here.
Practise this grammar point on paper: 19 exercises using only HSK 2 words, with an answer key on a separate page. Free to print and copy for your class.
Every example on this page uses only words up to HSK 2. Paste your own sentences into the level checker to see which words, if any, are above your level.