Possibility or Suggestion

[Jadoon 84]

While studying modals in a book written by nonnative speaker, I found that Might & Could are used for suggestion as well i.e.
You might/ could sleep in my room today.
You might / could have slept in my room. (Past)

The book says that could and might can be used for possibility in past as well. i.e.
He might / could have downloaded Windows7 in his home.

Question: Me, being a nonnative speaker, how will I come to know about a sentence if it means a possibility or suggestion.

Could you please clear my confusion?

with kind regards

[BobK]

Context.

If I say 'The door was unlocked, so that we might leave at any time', that's a possibility.

If I say 'Might I open the window?', that's a suggestion.

There's probably a rule - something to do with the tense of the verb in the main clause. But context works for me.

b

[Tdol]

How do you make a suggestion in the past, when there's no chance of it being taken up?

[BobK]

Exactly! There's the theoretical possibility of ambiguity, but I can't think of an example when the distinction is not perfectly clear in practice. To quote Lewis Carroll said 'Take care the sense and the sounds will take care of themselves'.

b

[Barb_D]

As an American, I wouldn't use "might" for past permission (as I saw in a recent post) or suggestion.

• You may leave the table when you've eaten your vegetables (current permission)
• Yesterday, you could have/were allowed to leave the table when... (past permission) - would not use "might have" there.

• You may have found a solution! (present possibility)
• Yesterday, you might have found the solution we have been looking for. It's still being tested.

• You may/might want to ask someone who knows about this stuff (present suggestion)

• May/Might I open the window? Request. American are less likely to use "might" here, I believe.