应该, 必须, 不要: should, must, don't

应该 is “should”, advice. 必须 is “must”, a rule, firmer. 不要 is “don't”, a ban. “Not necessary” is 不用, not 不必须.

Examples

你应该早一点儿睡觉。 Nǐ yīnggāi zǎo yìdiǎnr shuìjiào. You should go to bed earlier.
明天八点上课，你必须来。 Míngtiān bā diǎn shàngkè, nǐ bìxū lái. Class is at eight tomorrow. You must come.
上课的时候不要玩儿手机。 Shàngkè de shíhou búyào wánr shǒujī. Don't play with your phone in class.