Modal - 'can'

[Allifathima]

1. He can open the window.
2. He could open the window.
3. He can have opened the window.
4. He could have opened the window.
Are both correct?
5. They can't have arrived already, surely!
Is this sentence OK?

[Waiyin Cheng]

The 3 and 5 are incorrect.
The rest is fine but has different meanings.
Can always goes with bare infinitive verb (without to).

4. - He did not open the window in the past. Hypothetically, he should do it.
1. he has the ability to open the window
2. Not sure, It's hard to explain the meaning, But the grammar is correct.

I'm just a member, so do not take it for granted that my reply is all correct. I just try to help. Cheer !

[Beeesneees]

5 is correct.
_________________
"Inside every old person is a young person wondering what happened."
Terry Pratchett

[Allifathima]

Beeesneees,
Is it not possible to use 'can have opened' ?
Is it grammatically incorrect?
In sentence #5, "can't have arrived" is found used.
Please explain the grammar nuance involved in it.
Thanks.

[Beeesneees]

Context is everything.
'He can have opened the window' doesn't make sense.
I can't think of a situation where 'can have opened' or 'can have arrived' would be useful.

'He can't have opened the window', on the other hand, does make sense. This appears to be one of those incidences of something only appearing in the negative form.
_________________
"Inside every old person is a young person wondering what happened."
Terry Pratchett