modal verbs ( should, must, have to )

Should

Used to give advice. It indicates that something is good and therefore should be done. It is a recommendation, not an obligation. Tax is less powerful than "must" and "have to".

If you feel bad You Should Go To The Doctor

Also used to give an order but very polite.

Anyone who wants to enter the conference Should show his credentials

(It really is a must-show credentials is required to enter the conference, but is expressed in a rather coercive)

Construction: e ste modal verb is followed by the infinitive of the main verb without the particle "to".

Used in past, present and future.

I should have finished my report yesterday (past)

You should help me, I can not do it alone (this)

Tomorrow You Should parents call your (future)

The past is used to indicate something that should have been done and not done.

The negative form is: Should

You should not smoke

And the interrogative: Should + subject ...?

Should I wear a dark suit for the party?

Must / Have to

They express an obligation to do something.

You must come here Immediately (that's an order)

If you want to make a career in a company You have to speak Inglés (There is no alternative: either speak English or no career possible)

See the difference with the following prayer:

Nowdays it is very important to speak Inglés. You should do it.

Here is a tip, while the former was an obligation.

Moreover, when using "must" / "have to" the issuer is convinced that the obligation will be fulfilled, whereas when using "should" does not know if the board is going to continue or not.

Should we buy a new car (it is a desire, there is no assurance that we will do)

We must buy a new car (there is a good chance that we do)

We can use have to + infinitive, must + infinitive and should + infinitive to express obligation (something you have to do).

Present	Positive	Negative
have to / don’t have to	strong obligation (possibly from outside) Children have to go to school. (sometimes ‘have got to’)	no obligation I don’t have to work on Sundays. You don’t have to eat anything you don’t like.
must / mustn’t	strong obligation (possibly based on the speaker’s opinion) I must study today.	negative obligation You mustn’t smoke here.
should / shouldn’t	mild obligation or advice You should save some money.	mild negative obligation or advice You shouldn’t smoke so much.

Be careful about the difference between mustn't and don't have to!
Mustn't means it's not allowed, or it's a bad idea:

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

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

Past	Positive	Negative
had to / didn’t have to	obligation in the past I had to go to wear a school uniform when I was a child.	no obligation in the past We didn’t have to go to school on Saturdays.
must*	changes to 'had to'	-
should have + pp / shouldn’t have + pp	a past action which didn’t happen: the advice / regret is too late You should have gone to bed earlier, now you have missed the train.	a past action which didn’t happen: the advice / regret is too late You shouldn’t have taken that job., it was a bad idea.

* Remember ‘must have done’ is a modal verb of deduction or speculation, not obligation in the past. For example: Julie must have left. Her coat’s not here. See modals of probabilty for more information.