be supposed to do意为“应该、被期望、理应”，用来表示根据规定或传统习惯人们不得不做的事。时态、人称和句式的变化用be来体现，to为不定式符号，后接动词原形。
　　1. be supposed to do的肯定句式为：
　　Everyone is supposed to wear a seat-belt in the car.
　　每个人在小汽车里都应该系好安全带。
　　You’re supposed to start work at 8∶30 every morning.
　　你应该每天早晨8点半开始工作。
　　They are supposed to shake hands.
　　他们应该握手。
　　2. be supposed to do的疑问句式须将be的相应形式提到主语前。例如：
　　Is he supposed to get there at five？
　　他应该在5点钟到吗？
　　Was Lucy supposed to speak Chinese like that？
　　露西应该像那样说汉语吗？
　　What are you supposed to do？
　　你们应该做什么？
　　When were you supposed to arrive？
　　你们应该什么时候到？
　　3. be supposed to do的否定式，即在be动词后加否定副词not，此时相当于not be allowed to do，意为“不被许可、不应当”。例如：
　　You are not supposed to smoke here.
　　你不应该在这里抽烟。
　　We are not supposed to play football in the street.
　　我们不应该在大街上踢足球。
　　It is a secret still. I am not supposed to say.
　　这还是个秘密，我不能说出来。
　　4. be supposed to be的主动形式为“suppose sb. to be＋形容词／名词”。例如：
　　All the classmates suppose him to be stupid.（＝He is supposed to be stupid by all the classmates.） But in fact， he is a good student.
　　所有同学都认为他愚蠢，事实上他是一个好学生。
　　People in old times supposed the earth to be flat.（＝The earth was supposed to be flat by people in old times.） But now we know it is round.
　　古时候人们认为地球是平的，但我们现在知道它是圆的。
　　We suppose her to be an actress. ＝She is supposed to be an actress.
　　我们认为她是个女演员。
　　5. suppose作动词，意为“认为、猜想、想象”，suppose的后面接从句和to be作宾语。例如：
　　I suppose he is not yet thirty.
　　我猜他还不到30岁。
　　—Do you suppose it is going to rain？你认为会下雨吗？
　　—I suppose so. 我想会吧。
　　We all suppose Jenny to be the best student in our class.
　　我们都认为珍妮是我们班里最好的学生。
　　［注意］作此意讲时，suppose可用于简单答语中。常用于简单答语中的动词还有think，believe，expect，hope等。suppose引导的宾语从句如果是特殊疑问句，疑问词应提前。例如：
　　Who do you suppose is coming with Mr Yang？
　　你认为谁会和杨老师一块儿来？
　　6. suppose作动词时还有“假定、假使”之意，后接从句。suppose此时可当作if来用，往往表达想象中的条件，这样用时，这个动词相当于supposing。假如suppose作“如果”讲，则只能用supposing，不用suppose。例如：
　　Suppose （that） it rains， what shall we do？
　　假如下雨我们怎么办？
　　Let’s suppose （that） he is telling a lie.
　　姑且假定他在说谎吧。