Supposed to is part of a modal verb phrase meaning expected to or required to. Although suppose to crops up frequently in casual speech and writing, it should not be used in that sense. Suppose (without the d) should only be used as the present tense of the verb meaning to assume (something to be true).
When to use supposed to
To be supposed to is a common phrase that functions the same way a modal verb does. Modal verbs, also called auxiliary or helping verbs, add meaning to the main verb in a sentence by expressing possibility, ability, permission, or obligation. Supposed to, like have to, can fall into the “obligation” category.
I am supposed to
be home by midnight or my coach will turn into a pumpkin.
I am supposed to be home by midnight or my coach will turn into a pumpkin.
It can also be used to indicate what a person (or thing) is likely to do or is reputed to do.
She is supposed to
be the best poodle breeder in town.
She is supposed to be the best poodle breeder in town.
Whenever it is used in either of these senses, supposed to will be preceded by a form of to be and followed by a verb.
Were we supposed to
be here so early?
Were we supposed to be here so early?
The verb to suppose
On the other hand, the verb to suppose means to presume, in theory, that something is the case without certain evidence.
The committee supposes
that the decline in profits is due to a faulty product design.
The committee supposes that the decline in profits is due to a faulty product design.
As an imperative, it can mean the equivalent of what would happen if.
Supposed to, have to, and ought to
Some English language learners are easily confused by the modal verbs to be supposed to, have to, and ought to. While all three function similarly in a sentence, their meanings are subtly different.
To be supposed to do something implies that the subject is obligated and expected to do the main verb’s action, although there is a possibility it won’t happen.
Here, there is an expectation that the magician will be able to perform his trick correctly, but there is always a chance that he won’t succeed. The sentence conveys this expectation, and the reader must now wait to see if that expectation will be fulfilled.
Has to means the same as must; it implies that the subject has no choice about performing the verb’s action.
This sentence conveys that if the magician does not produce the rabbit, there will be no applause. He simply must do it to get the result he wants.
Ought to is more of a suggestion. It conveys what would be best for the subject to do, but the subject is not compelled to do it.
To ensure that he has a compliant, content rabbit in his hat, the magician should give his rabbit a carrot, although no one requires it of him. It would simply be a good idea.
If you only remember a single grammar factoid about suppose, let it be that you can confidently expunge “I am suppose to . . .” from your literary repertoire forever.