In the active voice, the subject is the actor or agent, and the object is the patient (the recipient of the action). In the passive voice, the subject is the patient and the object is the actor. In the active voice, the subject is doing what the verb says: I eat. In the passive voice, what the verb says is being done to the subject: I am eaten. The passive voice takes the past participle of the verb and puts it behind a helper verb, usually a form of be.1 No matter the tense, number, or person of that helper verb, the main verb remains the same. I am eaten, they are eaten, we were eaten, you will be eaten.
People don't think of supposed to as a passive voice construction, but that's what it is. To suppose2 is to assume or expect. When you say Joanna is supposed to be at the market, it means someone is supposing that she is there. It's clunky, but not incorrect, to say Joanna is assumed to be at the market. In the active voice, you could also say I suppose (or assume) Joanna is at the market.
The trouble is that supposed to is so common, it just feels like a modal verb. People use supposed to the way they use should, would, or might: Joanna might be at the market. When people change the tense of the sentence, they think they have to change the tense of suppose because it feels like it's the main verb. That's why people say *Was Joanna suppose to be at the market instead of Was Joanna supposed to be at the market. But suppose is not the main verb—the main verb is be.
Once you realize it's just an example of the passive voice, supposed to becomes clear:
is supposed to
was supposed to
will be supposed to
You can even put together complicated sentences like When Joanna reaches the campground, she will be supposed to have been to the market. That's unnecessarily complex, but notice that supposed didn't change. It wasn't supposed to.
But sometimes get.
When you suppose something, it is supposed—which is an adjective, kind of, that you can turn into the adverb supposedly. When you say Supposedly, Joanna is at the market, it means someone is supposing that she is. Some people say supposably for some reason, which means able to be supposed. But it's not as wrong as you might think it is.
I think that there is another meaning for “Joanna is supposed to be at the market” than that someone is supposing she is there. The someone has just received a report that Joanna, who is expected to be at the market for an important reason, is nowhere to be found! There is a factor of Joanna’s failure to fulfill expectations, not just a supposition. What do you think?
I think that there is another meaning for “Joanna is supposed to be at the market” than that someone is supposing she is there. The someone has just received a report that Joanna, who is expected to be at the market for an important reason, is nowhere to be found! There is a factor of Joanna’s failure to fulfill expectations, not just a supposition. What do you think?