be supposed to的用法
be supposed to do sth相当于should do sth或者是be expected to do sth.人称,时态和句式体现在be的变化上，to为不定式符号后接动词原形。

句式:
1)肯定句:be supposed to do sth
2)否定句:be not supposed to do sth相当于shouldn't do sth.或 be not allow to do sth.意为“不应当做某事;不被允许做某事”

3)疑问句:be supposed to do sth的疑问句是将be动词的相应形式提到主语前面。
Are you supposed to sleep at nine?晚上你应该在9点睡觉吗啊?
What are you supposed to do on weekend?在周末你应该干什么?