Use of Can and Could

We use can as a principal verb to express

1. Power or ability, ex- 

• I can talk in English.
• She is smart but she can’t go alone.
• She can study hard in exams.
• She can do it.

2. Possibility, ex- 

• This exam can be tough.
• You can easily complete this task.

3. Permission, ex- 

• You can go now. (you are permitted to go now.)
• Can I cook noodles? No, you can’t
• You are not allowed to do that.

We use could to show that something is possible, but not certain:

They could go there. 
They could be at the restaurant. 

could is more formal and polite than can:

Could I come inside?
Could I sit here?

Requests

We use could you, as a polite way of telling or asking someone to do something:

Could you give me this pen, please?
Could I have my phone, please?

can is less polite:

Can you give me this pen, please?

Offers

We use can I … to make offers:

Can I help you to complete your assignments?
Can I do to help you?

We sometimes say I can  or I could  to make an offer:

I can do that for you if you like.
I could give you a lift to the station.

Suggestions

We use could to make suggestions:

We could meet at the bar.
You could eat biryani today.

Could is also used for imaginary situations:

For example : 

• I wish I could be a Superman. 
• I wish you could understand her perspective.
• I wish I could date Rakhi.
• I wish you could see the reality.

Sometimes could is also used in conditional sentences:

• If you buy a lottery ticket, you could be lucky.
• If you arrange the medicines, we could save the patient.

We use can be/could be in the passive voice:

can be/could be + third form of the verb

ex- 

• This video can be downloaded from my youtube.
• The payment can be made by UPI.
• We could be charged for this mistake.