Modal verb a modal verb is a kind of auxiliary verb that is used to express modalities (the states or ‘modes’ in which a thing exists) such as possibility, ability, prohibition and necessity

What are the uses of modal verbs?
As we mentioned before, modal verbs can be used to express a number of different things. Let’s take a look through some of the main uses of modal verbs:
1) Modal verbs of prohibition
Modal verbs can be used to convey obligation and necessity within a sentence. These command-focused words are a useful way to direct a reader or listener with clear guidance for what to do in a given situation.
For example, saying ‘you must do your homework tonight’ is very clear, putting the importance of this action right at the top of the sentence.
Here are some examples of modal verbs of prohibition in action:

• 
  We must hurry if we are to get there on time.
  
• 
  She will have to eat all her dinner if she wants ice cream.
  
• 
  Jenny has got to learn how to tie her laces.
  

As you can see in these three examples, the modal verbs of prohibition can be used to describe an action happening in the present tense, present continuous, or future tense, depending on the situation. Words can be used to express this prohibition are‘must’, ‘have’, ‘should’, ‘can’ and ‘do’.
2) Modal verbs of suggestion
Modal verbs can also be used to give advice or to suggest an action. This is more suitable for situations where it might be considered rude to give an outright order, or for more conversational guidance.
Modal verbs that can give a suggestion are ‘could’, ‘should’, ‘ought to’, and ‘had better’. These examples get more strong in their urges as they go on. Compare the tone in the following sentences to see this effect in action:

1. 
   You could stay at home if it’s raining.
   
2. 
   You should stay at home if it’s raining.
   
3. 
   You ought to stay at home if it’s raining.
   
4. 
   You had better stay at home if it’s raining.
   

By the final sentence, it sounds less like advice and more like a command! This shows how modal verbs can affect our tone depending on the situation.