1. Explain it simply
应该 = should, 必须 = must, 需要 = need to.
Put the modal after the subject and before the main verb.
The modal never goes at the end: 你应该多休息, not 你多休息应该.
应该 is a suggestion; 必须 is a strong requirement.
2. Show examples
你应该多休息。
Nǐ yīnggāi duō xiūxi.
You should rest more.
should
我们必须努力。
Wǒmen bìxū nǔlì.
We must work hard.
must
你需要多喝水。
Nǐ xūyào duō hē shuǐ.
You need to drink more water.
need to
学生应该认真学习。
Xuésheng yīnggāi rènzhēn xuéxí.
Students should study seriously.
modal before verb
3. Share a tip
The modal sits between the subject and the verb - never drop it to the end of the sentence.