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Using must, have to, should, ought to, need and related expressions.
English uses must, have to, need to, should, ought to, and related expressions to describe obligation, necessity, recommendation, and advice.
These expressions differ in strength, source of obligation, time reference, and degree of certainty.
Must expresses strong obligation or a rule that the speaker considers necessary.
For example, You must wear a seat belt and We must finish this today.
Have to expresses necessity or obligation that often comes from external circumstances, rules, or requirements.
For example, I have to work tomorrow and Students have to wear uniforms.
Must and have to can both express obligation, but must often presents the obligation as coming from the speaker or the situation while have to often presents it as an external requirement.
Compare I must call my mother with I have to wear a uniform at work.
Need to expresses that an action is necessary or required.
For example, You need to submit the form today and We need to leave now.
Should is commonly used to give advice, recommendations, or mild obligation.
For example, You should get some rest and We should leave soon.
Ought to expresses advice, recommendation, or expectation and is often similar in meaning to should.
For example, You ought to apologize and We ought to be more careful.
Should and ought to usually express similar meanings, but should is much more common in everyday English.
Ought to is somewhat more formal and is less common in many varieties of spoken English.
Must not expresses a strong prohibition against doing something.
For example, You must not touch this button and Visitors must not enter this area.
Do not have to means that something is not necessary or required.
For example, You do not have to come early means that coming early is optional.
Must not means prohibition, while do not have to means lack of necessity.
Compare You must not park here with You do not have to pay today.
Need not expresses that an action is unnecessary.
It is a modal form and is especially common in formal British English. For example, You need not wait.
Need not and do not need to can express the same general meaning of lack of necessity.
Need not is more formal in many contexts, while do not need to is common in everyday English.
Had better expresses strong advice or a warning about what is advisable in a particular situation.
For example, You had better leave now and We had better call them.
Should not or should not expresses advice against doing something.
For example, You should not drive when you are tired and We should not ignore the warning.
Ought not to can express negative advice or a recommendation against an action.
For example, You ought not to speak to him like that. This form is less common than should not.
Past obligation is commonly expressed with had to rather than must.
For example, I had to leave early yesterday because I was ill.
Future necessity can be expressed with will have to, will need to, or other appropriate constructions.
For example, You will have to renew your passport next year.
Should have and ought to have can express advice or criticism about an action that should have happened in the past.
For example, You should have called me and She ought to have checked the details.
Should have + past participle can express regret, criticism, or an expectation that was not fulfilled.
For example, You should have told me earlier.
Must followed by have and a past participle expresses deduction about the past, while must have to expresses past or future necessity only through appropriate surrounding grammar.
Compare She must have forgotten with She had to leave early.
Advice is generally weaker than obligation, and the choice of modal changes the force of the statement.
Compare You should see a doctor with You must see a doctor.
English can distinguish between obligations imposed by the speaker and obligations imposed by external rules or circumstances.
Must often presents a personal or speaker-imposed obligation, while have to commonly presents external necessity.
Different modal expressions communicate different levels of force.
A rough scale may range from should and ought to through need to and have to to must, although context can change the strength.
The choice between must, have to, need to, should, ought to, and related expressions depends partly on register.
Have to, need to, and should are very common in everyday English, while ought to and need not may sound more formal.
Use obligation and advice expressions according to their exact meaning and grammatical pattern.
Common mistakes include confusing must not with do not have to, using must for all past obligations, omitting to after have to or ought to, and confusing should have with should.