Modals : Can or Could

                                                                Can or Could

                                                           Can or Could expresses 

• Ability in present and past

He can speak many languages. (Ability in the present)

He could speak many languages. (Ability in past)

I can walk this path at night.  (Ability in the present)

I could walk this path at night.  (Ability in past)

                                                         Can or Could for Permissions 

• To seek and give permissions

Can I go? (To seek permissions)

Can we buy the kit? (To seek permissions)

Yes, you can go. (To give permissions)

Yes, you can buy the kit. (To give permissions)

Could is used for general permissions in the past

He couldn't go for a movie, because his parents objected to it.

                                                            Can or Could for Requests 

• Requests

Informal request: Can you give me a pen?

Formal request: Could you give me a pen, please?

                                                   Can or Could are used for Possibilities

• Possibilities

It can be very cold here at night. (Possibility in present)

The storm could have destroyed the forest. (Possibility in past)

It can destroy your ambitions. (Possibility in present)

The war could have harmed wildlife. (Possibility in past)