The ad link the modal verbs the response words the interjection

Rules for the Modal Verbs Must and Have To
The modal verb must is used to express obligation and necessity.
The phrase have to doesn’t look like a modal verb, but it performs the same function. Have to can play the role of must in the past (had to) present, and future tenses.
These words can often be used interchangeably in English; the main difference between must and have to can be the emotional context.
Must means that the obligation to do something comes from the speaker. In other words, it’s not a rule. Have to implies that the obligation comes from someone else; it’s something the speaker can’t change. Additionally, must expresses the speaker’s feelings, whereas have to expresses, above all, an impersonal idea.
For example:

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  I have to pay the bills because my wife is ill.
  
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  I must stop smoking. It’s really bad for my health.
  

In the negative form, must expresses a strict prohibition: You mustn’t smoke here.
However, have to in negatives sentences expresses the idea that “you are not obligated to do something, but you can do it if you want to” and requires an auxiliary verb: You don’t have to pay for this.
Also, the word must expresses submission to various general laws; that is, something needs to be done, because it is accepted as proper in society.
Have to expresses submission to private “laws,” that is, you are following your conscience, moral principles, or duties.
For example:

• 
  We must pay taxes.
  
• 
  He has to tell her the truth.
  

Rules for the Modal Verbs May and Might
First of all, the modal verbs may and might are used to express that something is possible, and they are used both in the present and future tenses.
For example:

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  It may be true. = It might be true.
  
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  He may know. = He might know.
  
• 
  Take an umbrella with you. It might be raining later.
  
• 
  We haven’t decided yet where to go for our holidays. We may go to Ireland.
  

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