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Modal verbs
We use modal verbs to show if we believe something is certain, probable or possible (or not). We also use modals to do things like talking about ability, asking permission making requests and offers, and so on.
 
Modal Verbs
Modal Verb
Use
Examples
can
1. general ability
    (be able to do
    something)
 
2. request
He can walk and run after five operations.  
Can you hear me?
I can't hear you.
  
Can you do me a favour? (more informal)
could
1. general ability
    in the past
 
2. polite request
My grandmother could speak seven languages.
My grandmother couldn't speak seven languages.
  
Could you pass me the salt please?(more polite)
may
permission
 
May I ask a question?
  
must
necessity
 
A building must have a fire alarm.
You mustn't smoke here.