must have/ should have/ ought to have

[navi tasan]

1-He must have gone there at three o'clock.
2-He should have gone there at three o'clock.
3-He ought to have gone there at three o'clock.

Which of the above sentences:
A-means that he had to go there at three o'clock but didn't
B-means that he most probably/necessarily went there at three o'clock
C-could mean both A and B depending on the context.

[Tdol]

A- 2\3
B-1
C- none

[navi tasan]

Right.
But consider:

4-He should have arrived by now.
5-He ought to have arrived by now.

or even:

6-He should have arrived. ("by now" implied)
7-He ought to have arrived.

Don't "should" and "ought to" imply liklihood in these cases, where "by now" is stated or implied?
Aren't these sentences ambiguous?
He should have arrived by now, but he hasn't.
He should have arrived home by now. Give him a call.

[RonBee]

Should have and ought to have are used pretty much the same way, especially in conditional sentences. (I think should have is used more often.

I don't see any ambiguity in your sentences. They seem pretty clear to me.

:)

[Tdol]

Right.
But consider:

4-He should have arrived by now.
5-He ought to have arrived by now.

or even:

6-He should have arrived. ("by now" implied)
7-He ought to have arrived.

Don't "should" and "ought to" imply liklihood in these cases, where "by now" is stated or implied?
Aren't these sentences ambiguous?
He should have arrived by now, but he hasn't.
He should have arrived home by now. Give him a call.

They're not ambiguous- but they do mean different things. They'd be clear in context.

[navi tasan]

Thanks TDOL and RonBee.
I think as usual I have managed to create a lot of confusion for myself.
Let me reformulate my question:
Is this correct:
7-You should have finished your homework by now, but as usual, you have wasted a lot of time and have done very little work.

[Tdol]

That's correct.