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1 Compare:
 | Tim had to go to the dentist's. (Tim tuvo que ir al dentista.)
(Here, Tim went to the dentist's.) |
| Pam should have gone to the dentist's. (Pam tenía que ir al dentista.)
(Here, Pam did not go to the dentist's.) |
Normally didn't have to implies that the action did not happen, e.g.:
| We didn't have to pay to see the exhibition.
(No tuvimos que pagar para ver la exposición.) |
| Ron shouldn't have accepted the money. (Ron no debería haber aceptado el dinero.)
(This implies that Ron accepted the money.) |
2 Here are examples:
| You must leave early. (Tienes que salir pronto.) |
| Do we have to pay? (¿Tenemos que pagar?) |
| Have we got to pay? (¿Tenemos que pagar?) |
| You should have a hot drink. (Deberías tomar una bebida caliente.) |
| You ought to have a hot drink. (Deberías tomar una bebida caliente.) |
3 Here are examples:
| Tom's away today, but I must talk to him tomorrow.
(Tom no está hoy, pero tendré que hablar con él mañana.) |
| A big order's come in so we'll have to work on Saturday.
(Ha llegado un gran pedido y, por lo tanto, tendremos que trabajar el sábado.) |
| You mustn't forget to buy some bread this evening.
(No te olvides de comprar pan esta noche.) |
4 Compare:
| You mustn't drink and drive; it's dangerous.
(No debes beber y conducir, es peligroso.) |
| Car drivers don't have to wear helmets.
(Los conductores de coche no tienen que llevar casco.) |
In other words, mustn't expresses obligation not to do something, but don't have to expresses that there is no obligation.
| You shouldn't eat so much.
(No deberías comer tanto.) |
| You oughtn't to eat so much.
(No deberías comer tanto.) |
In other words, should and ought are synonymous, but ought needs to.
5 An example:
| With the new law my son won't have to do military service.
(Con la nueva ley, mi hijo no tendrá que hacer el servicio militar.) |
6 Here are examples:
| After my eighteenth birthday, I was allowed to come home late.
(Después de cumplir dieciocho años, me dejaban volver tarde a casa.) |
| A: Can I sit here? (¿Puedo sentarme aquí?) |
| | B: You can sit wherever you like. (Puedes sentarte donde quieras.) |
| C: May I smoke here? (¿Puedo fumar aquí?) |
| | D: Yes, you may. / No, you may not. (Sí, sí que puedes. / No, no puedes.) |
Notice that may expresses permission of the speaker towards the other person or, in a question, asks the permission of the other person. Consequently, we cannot say may you: May you come tomorrow?
Notice that can is also used for capacity. Consequently, the following sentence has two interpretations.
| Can you lift heavy things?
¿Puedes (= eres capaz o te dejan) levantar cosas pesadas? |
In order to avoid this ambiguity, we can express permission with be allowed to:
| Were you allowed to lift heavy things?
(¿Te dejaban levantar cosas pesadas?) |
| Are you allowed to lift heavy things?
(¿Te dejaban levantar cosas pesadas?) |
| Will you be allowed to lift heavy things?
(¿Te dejarán levantar cosas pesadas?) |
7 Examples:
| We need to practise a lot.
(Necesitamos practicar mucho.) |
| They don't need to pay everything now.
(No es necesario que lo paguen todo ahora.) |
Notice that there is a difference in meaning between the past structures with need:
| Peter needn't have waited for me. (No era necesario que Peter me esperara.)
(This implies that Peter waited.) |
| Mary didn't need to take her passport. (Mary no necesitaba llevar el pasaporte.)
(Here, there is no implication either that Mary took her passport or that she didn't.) |
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