Chinese has three common words for what you need to do. 得 (děi) is the everyday spoken "have to". 必须 (bìxū) is a firm "must", often for rules. 应该 (yīnggāi) is "should": the right or sensible thing to do.
Subject + 得 + verb.
This 得 is read děi, not de. It is the usual word in conversation when something needs to be done, often because of the time or the situation.
Subject + 必须 + verb.
必须 is stronger than 得 and sounds more serious. You often see it in rules and instructions, and it is the word to use in writing.
Subject + 应该 + verb.
应该 gives advice or says what is right. With 不 in front, 不应该 means "shouldn't". 应该 can also mean "should" in the sense of "probably": 他应该到了 is "he should be there by now".
The most common mistake is the negative. To say "don't have to" or "don't need to", use 不用 for both 得 and 必须. 不得 does not mean "don't have to", and 不必须 is wrong. Note the tone change: 不用 is búyòng.
Practise this grammar point on paper: 16 exercises using only HSK 3 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 3. Paste your own sentences into the level checker to see which words, if any, are above your level.