Karshi branch of tuit named after Muhammad al-Kharazmi practical work

Possibility
You can use can or could to say whether something is possible.

• 
  You can call this number any time before 10pm.
  

In this sentence, the phrase can call means that it is possible to call the number so long as it is before 10pm. If the time is after 10pm, your call will not go through. Better check your watch!
The phrase you use will depend on which modal verb you’re using and what tense you’re writing in. For example:

• 
  You could watch the game on the big screen.
  

Here, we use could to suggest that this is one possible option of several.
Here are some more examples of using can and could to say if something is possible in a sentence:

• 
  I can’t believe what I’m hearing!
  
• 
  Yes, you could take a trip to Sacramento this weekend as you have vacation time left.
  
• 
  He couldn’t understand it if he tried.
  

Permission
You can also use modal verbs to ask for something, or to give or deny someone permission to do something.

• 
  Can you help me with this?
  

This question is asking for a response that will also use a modal verb (Yes, I can or No, I can’t).
Could also works in the same way here:

• 
  Could you help with this?
  
• 
  Could I borrow a pencil please?
  

When we answer these questions, can and could are used to give permission, and can’t and couldn’t are used to deny it, like this:

• 
  I can’t help with that, you have to do your homework on your own.
  
• 
  Yes, you could borrow a pencil.
  

In the first sentence, the speaker doesn’t have permission to help the asker. They’re trying to cheat!
Suggestions
You can use the modal verb could to suggest things someone is able to do out of a range of options.

• 
  You could take the express bus.
  

In this sentence, the phrase could take not only means that it is possible to take the express bus, but that it might be a useful or good thing for someone to do.