Could + perfect infinitive can be used instead of may / might + perfect infinitive. -
I wonder how Tom knew about Ann’s engagement.
-
He may / might / could have heard it from Jack.
In the interrogative either of them can be used but in negative the meanings are different.
-
Ann might not seen Tom yesterday.
-
Ann couldn’t seen Tom yesterday.
Subject + can can mean it is possible or circumstances permit.
-
You can ski. There is enough snow.
-
Can you get to the top of mountain in a day? It’s possible.
-
Measles can be quite dangerous.
