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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