MODAL VERBS 2ND ESO

1-Modals are different from normal verbs:

A: They don't use an 's' for the third person singular.

B: They make questions by inversion ('she can go' becomes 'can she go?').

C: They are followed directly by the infinitive of another verb (without 'to').

In this unit we are going to learn the uses of: SHOULD, HAVE TO AND MUST

A-Obligation and Advice

• Children must do their homework. (OBLIGATION / STRONG ADVICE) * We can also use HAVE TO for obligation (only in affirmative)

Native speakers in general USE have to for external obligations (rules, laws, schedules...) and must for personal obligations (things that you think that are important)

• You should/ shouldn't stop smoking. (ADVICE) (it can be used in affirmative and negative)

BE CAREFUL 

In the third person singular have to and don't have to experience changes. See the examples below:

- He has to be 18 to drive a car.

- She doesn't have to wear a school uniform.

BE CAREFUL about the difference between mustn't and don't have to!

B-PROHIBITION

Mustn't means it's not allowed, or it's a bad idea:

• You mustn't eat so much chocolate, you'll be sick

C- LACK OF NECESSITY (SOMETHING IT IS NOT NECESSARY)

Don't have to means you don't need to do something, but it's fine if you want to do it:

• I don't have to get up early at the weekend(of course, if I want to get up early, that's fine, but I can stay in bed if I want).

AS A SUMMARY:

HAVE TO: external obligation or strong advice

DON'T HAVE TO: lack of  necessity

MUST: strong obligation, internal obligation

MUSTN'T: prohibition

SHOULD: advice/ recommendation

SHOULDN'T: advice/recommendation
*IMPORTANT - You can watch this video to see the differentes between the three modal verbs. If you go directly to the minute 8: 50 you  Will find a summary of the uses of each verb.
https://www.youtube.com/watch?v=POFfLs0Uaio&feature=emb_logo