My explanation to a class might go like this:

"Suppose" has the primary meaning of *assume* or *speculate*. A *supposition
*, after all, is an assumption or speculation. In the active voice, we say
things like these:

   - People suppose that I am a tough guy, but I'm really not.

Sentences like this have passive equivalents:

   - I am supposed to be a tough guy, but I'm really not.

In the passive, in addition to *assumed*, "supposed" has broadened to
acquire the second meaning of *expected*, so that the following can have two
different meanings:

   - I am supposed to be a tough guy,

This can mean either that I am mistakenly believed to be a tough guy or that
I am expected/required to be a tough guy. The latter, unlike the former
meaning, has no active equivalent, so that "People suppose me to be a tough
guy" can only mean that they assume this; it cannot mean that they expect or
require it. The second meaning of the passive form is now used more
frequently than the first, so that "Students are supposed to hand in their
homework on time" can only mean *expected*, never *assumed*.

Dick

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