A few months back...no, wait, over six months back--see this post...I listened to an audio version of the Josephine Tey novel The Franchise Affair. It involves someone falsely accused of a crime, and early in the story there's an exchange between the accused and her lawyer which goes something like this:
What have you done?
I haven't done anything.
Well, what are you supposed to have done?
This use of "supposed," precisely in its strict sense, referring to something believed but with a degree of uncertainty, similar to "alleged" or "conjectured," caught my ear, because as often as not today we don't use that ordinary sense. And I would bet that its most frequent use is to convey something similar to "required" or "commanded," or at least "intended." "You're supposed to..." frequently means "you must."
Tey's sentence above, for instance, in isolation, would be most likely to refer to something meant to be done but not in fact done: "what should you have done?" or "what are you expected to have done?" But in common usage "You're not supposed to park here" means "you must not park here."
A couple of other words or phrases have taken on that imperative sense that they are not, well, supposed to have. It's a curious thing that I began to notice a long time ago. Frequently the imperative sense is a matter of word order in the sentence. Consider:
You have this form to fill out.
You have to fill out this form.
It would be better for you to fill out this form.
You'd better fill out this form.
The second sentence of each of those pairs is essentially a command: "You must fill out this form." "You'd better" is weaker, a command that's a little short of "must" but with an implied threat for non-compliance. I associate it first with children, especially girls: "You better give it to me," often followed by "or I'm going to tell Mama."
These things trouble me a little because they remind me that any particular language at any particular time is a sand castle. I spend a lot of time writing, and most of it now is poetry, in which every single word is to the best of my ability very precisely chosen and placed. Oh well, it isn't likely to be read very much, so it won't matter much when my words
Decay with imprecision, will not stay in place,
Will not stay still.