1

I know that "would have" is usually used in a hypothetic situation.

For example, "I would have done that had I been able to" is not a real situation but a hypothetic scenario.

However, other similar structures, such as "might have" and "could have" can also be used to mean "assumption" or "conjecture".

For example, "He might have stolen the money", means that someone might have been the thief.

So, I would like to know if "would have" can be used in the same way.

For example, can I say "I don't know who would have done that except John" or "Only John would have done that"

Any advice and opinions are welcome.

Thanks in advance!

2

Yes, you can say:

  • "I don't know what time it was exactly, but it would have been about eight in the evening."

would
6.Used to express presumption or expectation:
That would be Steve at the door.
American Heritage Dictionary

5.Expressing a conjecture, opinion, or hope.
‘I guess some people would consider it brutal’
Oxford Living Dictionaries

All of the dictionary examples give simply "would", not "would have", however one example in Macmillan Dictionary gives one, though in question form.

1.used for saying or asking what someone thinks about a possible situation

Where would he have hidden the keys?
Macmillan Dictionary

The answer to that question is obviously "He would have hidden the keys..." (here/there).

You can say both:

  • Someone just knocked on the door. That would be Steve.

or

  • Someone knocked on the door earlier. That would have been Steve.

If you search "probably would have" in Google Books, you will find many examples of the type you're talking about:

And you probably would have answered that you didn't enjoy yourself here.
- Mortal Gods, 2014

A subsequent generation of justices, who probably would have found segregation even more abhorrent than their predecessors had, would have been sorely tempted to apply the ascendant national norm against segregation to shrinking numbers of holdout states.
- Brown v. Board of Education and the Civil Rights Movement, 2007

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