Modal Verbs of Obligation

In English there are various ways to describe rules and obligations, and it can be tricky to understand which way is right for each situation. 

• need to, 
• must and 
• have to are all used to express strong obligation. 
• All three have equal strength. 
• ‘shall’ is used to describe formal obligations.

Here is a description of each modal verb that we use to express obligation and how and when to use them.

• need to
• must and 
• have to are all used to express strong obligation. 
• All three have equal strength.

Must

• ‘Must’ is the basic and most direct way of saying something is compulsory. 
• It means you have no choice but to do (or not do) an action. 
• It is formal
• We can only use ‘must’ in the present form 
• to describe a general or permanent obligation, 
• You must wear a seat belt at all times.
• Travellers must show their passport before boarding a plane.
• You must use a pencil on the test.
• or an obligation in the near future.
• I must go to work today.
• My tooth is sore. I must go to the dentist.

Must Vs have to

Must and have to express obligation or necessity, but there are some small differences.:

• must is used for internal obligations, and have to is used for external obligations.
• 'Have to' shows that the obligation comes from someone else, not the speaker. This is usually referring to a rule or law.
• To travel, you have to carry a passport.
• I have to work on Sunday..
• 'Must' shows us that the obligation comes from the speaker.
• I must hand in these articles by tomorrow.
• I must call my freinds.
• the difference between 'have to' and 'must':
• If I want to lose weight, I must start doing more exercise.(I have no choice)
• The doctor told me to lose weight so I have to go to the gym (it’s my decision)

The negative forms of Must and have to

• Mustn't expresses prohibition
• You must not eat in the computer room. 
• (Food in the computer room is prohibited.
•  Don't have to expresses the absence of obligation or necessity:
• I don’t have to work today. 
• (Working is not necessary.)

if you are still a bit confused ...

Here's an example you can remember:
In a non-smoking area you mustn’t smoke, but in a smoking area you don’t have to smoke but you can if you want to.

Need’ as a modal

• As a modal verb, ‘need’ is most typically used in negative sentences to say there is no obligation. It expresses absence of necessity or obligation
• She need not arrive (needn’t arrive) so early.
• I don’t need to pay now. I can pay later.
• But to say that there is an obligation not to do something, use “mustn’t”.
• She mustn't use such horrible language.
• Tom. You mustn't play with fire.
• You mustn't drive more than 25 mph in this zone.
• We can also use “can’t”:
• “You can’t go out tonight. You’ve got homework.

Should is a weak obligation, and we use it to give advice.

• I think you should go to the doctor (I think it is a good idea, that's my opinion)
• You shouldn't smoke so much (I think it is not a good idea, that's my opinion)

shall’ used to describe formal  obligations

• Applicants shall provide self attested copies of their certificates to prove their qualification
• Employees shall provide a medical certificate for sick leave.
• you shall abide by the law.
• There shall be no trespassing on this property.
• Students shall not enter this room.