Must and have to

Course materials often teach that have to expresses 'external' obligation (obligation which is imposed by regulations, conventions or by somebody else's will) whereas must expresses 'internal' obligation (obligation which is imposed by the speaker).

You have to declare everything in your tax return. (external obligation)

You must try to get here earlier in future. (internal obligation imposed by a teacher on a pupil.)

This distinction may provide learners with a useful rule of thumb to help them to choose an acceptable form. However, many people don't make this distinction. Some people rarely use must to express any form of obligation, reserving it to express logical deduction (e.g. It must be later than I thought.) or advice (e.g. It's a brilliant film. You must see it.).

Some learners use must to express any degree or kind of obligation. In the case teachers may want to 'ban' it in order to promote some of the alternatives which more often express obligation (e.g. have to, should, ought to and had better).

Needn't and don't have (need) to

Course materials often teach that we use needn't to express 'internal' obligation and don't have (need) to to express 'external' obligation. This is similar to the 'internal/external obligation' distinction between must and have to.

You needn't stay after 6.00. (The person who says this has the authority to allow the employee to leave.)

You don't have (need) to clean the tools each time you use them. (The person who says this is referring to external conventions or regulations.)

This distinction provides a useful rule of thumb for learners, but in fact many people use only one of them regardless of whether the obligation is 'internal' or 'external'. Other people use them interchangeably.

The meaning of don't have to and don't need to often overlaps, but some people choose don't need to rather than don't have to in order to give permission not to do something:

You don't need to wait for me.

Needn't have and didn't have (need) to

Course materials often teach that needn't have refers to something which took place but was unnecessary, and that didn't have (need) to refers to something which was unnecessary and so didn't take place:

The flight was delayed for 8 hours so I needn't have got up so early.

I didn't need to go into work and so I spent the morning catching up at home.

In many varieties of English (AmE) didn't need to is used in both these contexts.

Mustn't and don't have to

The negative form of must and the negative form of have to have completely different meanings. Mustn't expresses an obligation not to do something. Don't/doesn't have to expresses an absence of obligation.

You mustn't eat anything for 12 hours before the blood test. (negative obligation)

Everything is ready so you don't have to be here early. (absence of obligation)

Some people use haven't to to express negative obligation, but this is not standard use.

(*) You haven't to park on double yellow lines at any time.

Hypothetical meaning

Would

Some course materials teach would as a modal verb only in conditional sentences with a clause beginning ‘if…’:

I would pay someone to do my cleaning if I could afford to.

In fact we frequently use would without if. This is often to:

• speculate.

• express hypothetical meaning.

• describe what we are imagining.

I would never consider changing career.

Could and might

We also use could and might to express hypothetical meaning. Could refers to ability and might to possibility.

I could never run a marathon now.

Don't eat. You might feel sick again.

Logical deduction

Will (‘ll), must and should

In its weak or contracted form we use will (‘ll) to express logical deduction when we are 100% certain.

Can you answer the phone? It 'll be Mum.

We also use must to express 100% certainty {It must be the battery...).

They sometimes teach to use will rather than must when our deduction is based on our knowledge of typical or repeated behaviour оr performance. However, will and must are often interchangeable.

Should usually expresses greater uncertainty, e.g. It should be Mum (but might not be).

Could and might

There is little difference between could and might:

Don't eat that. It could/might be poisonous.

Can't and mustn't

In most varieties of English, the opposite of must to express logical deduction is can't:

It must be six о 'clock. It can't be five о 'clock.

Time reference

Past

Could is the only 'pure' modal verb that we can normally use on its own to refer to past time.

Present	Past
She can swim.	She could already swim when she was six.

We use could to refer to the past only for general abilities. For specific events we have to express this in another way (e.g. She managed/was able to prise the door open, not * She could prise the door open.).

We can use other 'pure' modal verbs (and also ought to) to refer to past time by adding have + past participle:

I must have forgotten to lock the door.

She could have found the note.

You ought to have spent the evening resting.

Future

Modal verbs can normally refer to either the present or the future.

Present	You should try to exercise more control over the children.
Future	You should really try to visit us next year.

Sometimes we choose between a 'pure' modal verb and a semi-modal verb that has an explicit future form in order to make a subtle distinction:

I can finish the work tomorrow. (The ability exists now.)

I'll be able to finish the work tomorrow. (Something prevents me from being able to finish it now.)

Future arrangements and temporariness

We use modal verbs with a form of be and an -ing form to express meanings we normally associate with continuous forms of the verb.

Future arrangement	They should be recording another programme tomorrow.
Temporary activity in progress at a fixed point in time	Oughthe to be drinking so much?

Reported speech _;

We generally use could, might, ought to, should and would in reported speech just as we do in direct speech. Can and may frequently change in reported speech (can→could; may→might). We sometimes use had to instead of must.

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