Must/mustn”t

Must/mustn”t
Guía del uso del must y mustn’t.
MUST es un verbo 'modal'. Los verbos modales en inglés son verbos que se usan para expresar diferentes funciones como obligación, habilidad, opinión...etc. Se usan siempre con otros verbos.
Se usa: para hablar de obligación en inglés.

Se forma: con el sujeto + must + verbo infinitivo

AFIRMATIVO
I must study
You must study
He/she/it muststudy
We must study
They must study

NEGATIVO
I mustn´t be late
You mustn´t be late
He/she mustn´t be late
We mustn´t be late
They mustn´t be late

INTEROGATIVO
Must you travel to Barcelona tomorrow?
Must we travel to Barcelona tomorrow?
Must they travel to Barcelona tomorrow?
Se usa: 

1. Cuando algo es necesario
ejemplo: You must have a passport to to travel abroad. (Debes tenerun pasaporte para viajar al extranjero).

2. Para expresar lo que alguien cree que es necesario (opinión)
ejemplo: You must tidy your room (debes ordenar tu habitación).

3. Para prohibición en la forma negativa - Must not ó mustn´t - para algo que no debes hacer.
ejemplo: You mustn´t be late. (No debes llegar tarde)
Respuestas cortas:
Must you wear long hair?
Yes, I must.
No, I mustn’t.Have/don”t have to
* Have to is a construction (structure). It is not a modal verb. We use auxiliary verbs do and does to form questions and negatives.
Examples:

Positive
I have to work very unsociable hours.
She has to wear a uniform.

Negative
I don't have to work at weekends.
John doesn't have to do any work at home.

Question
Do I have to pay in advance?
Does your fatherhave to travel a lot at his job?
* With have got to negatives and questions are formed differently.
Examples:

Positive
I've got to work very unsociable hours.
She's got to wear a uniform.

Negative
I haven't got to work at weekends.
John hasn't got to do any work at home.

Question
Have I got to pay in advance?
Has you father got to travel a lot at his job?
Should/shouldn”t
*We use should and shouldn't to give advice or to talk about what we think is right or wrong.
* You should means something like I think it is a good idea for you to do it.
* You shouldn't means something like I think it is a bad idea for you to do it.
* Should is used to express the opinion of a speaker and often follows I think or I don't think.

Examples
     You look tired. Ithink you should take a few days off.

     Alice works very long hours. She should to talk to her boss.

     - I have an English test tomorrow.
     - I shouldn't worry if I were you. You have worked really hard.

     - I never have enough money.
     - I don't think you should go out so much.

Should - Quick Grammar Note
To give advice to someone you can also say:
* I should do itif I were you.
* I shouldn't be so rude, if I were you.
When you regret not doing something in the past, you can say:
* I shouldn't have spoken to him like that.
* I should have apologized earlier.
may/may not

Affirmative: may | Negative: may not | Interrogative: May ...? |
I may finish tomorrow
Puede que termine mañana (quizá) | I may not work tomorrow
Puede que no trabajemañana | May I use your telephone?
¿Podría usar su teléfono? |
You may be wrong
Tú podrías estar equivocado | He may not be right
Quizá él no tenga razón | May you tell me something?
¿Podría Ud. decirme algo? |
He may go to Paris tomorow
Él puede que vaya a París mañana | They may not know that
Ellos quizá no sepan eso | May I ask you a question?
¿Podría hacerle un pregunta? |
That maybe a mistake
Eso podría ser un error | It may not be so easy
Puede no ser tan fácil | May I interrupt you?
¿Puedo interrumpirlo? |
It may be very raining
Podría estar lloviendo | It may not be a good idea
Podría no ser una buena idea | May we suggest something?
¿Podríamos sugerir algo? |
That may be a good sign
Esa podría ser una buena señal | It may not be permitted
Quizá no esté...