SUPPOSE AND BE SUPPOSED TO - Learn the difference

 Suppose and be supposed to are used very often in English. Do not confuse them. 

The structures and meanings are not the same.

Suppose as a verb has a number of different meanings and uses.

Suppose can mean 'to think that something is true or probable'. 

• I suppose he is telling the truth.
• This situation will continue, I suppose.
• I don’t suppose he will let you buy him dinner.
• I supposed them to be married.

It is also used in short answers to refer back to something that has already been mentioned.

• ‘Do you think we will make it in time?’ I suppose so.
• ‘Shall we go to the party?’ I suppose.
• Will Jeremy attend the conference? I don’t suppose so/I suppose not. (If the answer is negative, both structures are possible here.)

Suppose is also used when you agree to something with reluctance.

• ‘Can I use your bike?’ ‘I suppose so.’ (= Yes, but I'm not happy about it).

Suppose is used in making polite requests. In this sense, we are not sure that we will get a positive answer.

• I don't suppose you could lend me your car this evening? [=Could you lend me your car this evening?]
• I don’t suppose you could give some advice on it, could you?
• I don’t suppose I could have a look at the photo?

Conjunction

Suppose/supposing = what would happen if…?

Suppose or supposing can also be used as a conjunction when we are asking about the consequences of an imagined situation. 

• Supposing there is no money left, what will we do then?
• Suppose you won the lottery, would you stop working?
• Suppose no one had been there? 

Suppose/supposing = what if…?

What if is also used instead of suppose or supposing to imagine possible situations

Be supposed to

Be supposed to is different from suppose.

be supposed to + infinitive = should

The structure be supposed to +infinitive is used to say what someone should do according to a law, rule or custom. It refers to what is expected or required. 

• I explain what you’re supposed to do.
• You’re supposed to be ready for class.
• Are we supposed to carry ID at all times?
• The flight was supposed to leave at 6.
• You were supposed to be here two hours ago!

In the past, be supposed to refers to things that didn’t happen for some reason.

The expression be supposed to refers to what is believed to be true by many people. 

• He is supposed to be the best player in the team. [Many people believe that he’s the best player in the team]
• The prison was supposed to be haunted.
• This spray is supposed to kill the weeds.

The negative structure of be supposed to is used to say what is not allowed.

• You're not supposed to park here. [=Parking is not allowed here.]
• You're not supposed to smoke in the building. [Smoking is prohibited in the building.]