Gramática inglesa de nivel medio paso a paso (Intermediate English Grammar Step by Step)

1. Can expresa habilidad en el present, y nunca cambia de forma. En pasado, se emplea could.
   She can drive a lorry. (Ella sabe conducir un camión.)
   They could swim like a fish when they were kids. (Sabían nadar como un pez cuando eran niños.)

   Con los otros tiempos verbales, usamos be able to, ya que can es un verbo defectivo1, esto es, carece de algunas formas verbales.
   I have been able to drive since I was eighteen (years old). (present perfect » Sé conducir desde los 18 años.)
   She will be able to lend you a hand when she comes. (futuro » Podrá echarte una mano cuando venga.)

   La negativa se hace con can't (forma contracta) o cannot (forma sin contraer). Nótese que cannot se escribe junto. Para la interrogativa se pone can delante del sujeto.
   He can't/cannot play the guitar. (No sabe tocar la guitarra.)
   Can he play the guitar? (¿Sabe tocar la guitarra?)

   En cuanto a could, la negativa es could not; y para la interrogativa, se coloca could delante del sujeto.
   She could not/couldn't play chess as a girl. (No sabía jugar al ajedrez de niña.)
   Could she play chess as a girl? (¿Sabía jugar al ajedrez de niña?)

a  They __________ (not) sing at all, so they will not __________ sing in the school choir.
b  I __________ learn languages very easily when I was at school.
c  "__________ you speak French?"
   "I __________ speak it as a child, but I _____(not) now."
d  We will not __________ help you if you don't tell us the truth.
e  She hasn't __________ speak since her accident.

2. Could únicamente es posible para indicar habilidad general en el pasado, en la negativa o con verbos como "see" o "hear", es decir, con verbos de percepción. Si nos referimos a una ocasión específica, empleamos was/were able to.
   As a child, I could2 run and jump all day without getting tired. (habilidad general » De niño, podía correr y dar brincos todo el día sin cansarme.)
   When she entered the house, she could smell gas. ("Smell" es un verbo de percepción » Cuando entró en la casa, olió a gas.)
   I lost my wallet in the street, but I was able to find. (en una ocasión en particular o específica » Perdí la cartera en la calle, pero pude encontrarla/me las arreglé para encontrarla/la encontré.)
   I lost my wallet in the street, but I couldn't find it. (negativa » Perdí la cartera en la calle, pero no pude encontrarla/no la encontré.)

a  As she entered her room, she __________ perceive an unpleasant odour. Her dog had done its business on the carpet.
b  Almost everybody failed the exam, but my son __________ pass it.
c  He __________ (not) do anything to save them: they had already drowned when he got there.
d  When I was at university, I __________ spend a lot of time reading, but now I can't.
e  Last year, he __________ break the world record for javeling throwing.

3. Para pedir permiso, se emplea can (o más formalmente, could o may); para darlo, can o may (formal). Véanse los siguientes ejemplos:
   "Can/Could/May I go to the loo?" (¿Puedo ir al servicio?)
   "Yes, of course!" (Sí, por supuesto!)
   You can/may go to the cinema with your friend, but don't come home late. (Puedes ir al cine con tu amigo, pero no llegues a casa tarde.)

   Can y may no poseen todas las formas verbales. Por lo tanto, recurrimos a la perífrasis be allowed to cuando can o may no sean posibles. Obsérvense los siguientes ejemplos:
   We have been allowed to use this swimming-pool since we moved into this house. (Se nos ha permitido usar esta piscina desde que nos mudamos a esta casa.)
   She used to be allowed to arrive home after midnight. (Se le dejaba llegar a casa después de medianoche.)
   From next week on, you will not be allowed to smoke/you can't smoke in your bedrooms. (A partir de la semana que viene, no se (os) permitira fumar/no podéis fumar en las habitaciones.)
   He could/was allowed to3 stay up until midnight when he was twelve. (Se le dejaba estar levantado hasta la medianoche cuando tenía doce años.)

a  "__________ I sit down?"
   "Yes, please do."
b  You've been a good boy, so you __________ telly tonight.
c  We haven't __________ park in this street since last year.
d  She __________ do what she wanted when she was living with her grandparents.
e  "__________ I go with you, daddy?"
   "Yes, sweetheart!"

4. May también puede significar "es posible que" o "puede que":
   We may visit her this afternoon. (Puede que/Es posible que la visitemos esta tarde.)

   La negativa se hace con not detrás de may, y normalmente sin contraer:
   She may not4 come tomorrow. (Puede que no venga mañana.)

   En la interrogativa, utilizamos do you think...?:
   Do you think (that) she will come tomorrow? (¿Crees que vendrá mañana?)

   Si nos referimos a una posibilidad general, utilizamos can; en el pasado, could:
   The sea can often be dangerous. (El mar puede ser a menudo peligroso.)
   When she was a child, she could be very naughty at times. (De niña/Cuando era niña, podía ser muy traviesa/mala a veces.)

   Could y might indican que la posibilidad es más remota o improbable:
   She could/might do it. (Puede que lo haga, pero no lo creo muy probable.)

a  We __________ go swimming this afternoon. If so, would you like to come?
b  "He hasn't studied at all."
   "Well, still, he __________ pass."
   "If you say so!"
c  The bite of this snake __________ often be fatal.
d  "I __________ (not) tell them. I don't know what to do. Shall I tell them?"
   "Well, this is up to you!"
e  "__________ it will rain this afternoon?"
   "I really don't know!"

5. Observe que un verbo modal + have + participio de pasado se refiere al pasado:
   She may/might have had an accident. (Puede que haya tenido un accidente.)
   You could have told me that he was not going to come! (¡Podrías haberme dicho que no iba a venir!)
   She can't/couldn't have done5 such a thing. (Es imposible que ella hiciera tal cosa/Ella no pudo haber hecho tal cosa.)

a  "The Smiths were coming to the barbecue, but haven't arrived yet, and it's getting very late."
   "They __________ have forgotten to come."
   "I don't think so. They love barbecues. They __________ have had a puncture."
   "A puncture _____(not) have delayed them so much!"
   "Let's phone them to see what has happened to them."
   "OK!"
b  You __________ have told me that the exam was postponed. I spent all night long studying!
c  They __________ (not) have talked to him last night: he's abroad!
d  He __________ (not) have bought a Rolls-Royce. He's as poor as a church mouse.
e  You __________ have taken a taxi! Why didn't you take a taxi?

6. Ejercicio de revisión.
a  "__________ you dance a walts?"
   "No, I'm afraid I _____(not)."
b  That year I __________ get the gold medal. It was the happiest day of my life.
c  We __________ tour the world next summer, but still don't know for sure.
d  Sometimes he __________ be very sweet, but he used to be very strict.
e  That place __________ be very dangerous. So I don't want you to go there again.
f  He __________ fail, but I think he'll pass.
g  You __________ (not) pick your nose in front of everybody. It's not appropriate for a young lady.
h  "__________ they'll let you down?"
   "No, I don't think so."
i  They __________ (not) have read the book, or they would know the plot.
j  "I __________ play draughts very well. Shall we have a game now?"
   "I'm very sorry, but I ­_____(not) play draughts. What about a game of cards, instead?"
k  "__________ I go to tonight's concert, daddy?"
   "Yes, you __________ as long as you don't get home very late."
l  I __________ see that something fishy was going on, so I decided to leave at once.
m  She used to __________ cheer us up in no time at all. We miss her a lot.
n  "__________ I speak, sir?"
   "Yes, go ahead."
o  I'm afraid to tell you that Mr Turner won't __________ attend the meeting. He's got a terrible cold.
p  Well, they __________ help you, but this is very unlikely.
q  He hasn't __________ see her since she was in hospital. He's been very busy lately.
r  We had better6 stay here, as it __________ rain this afternoon.
s  There __________ be another strike next week, so we'd better cancel our flight.
t  You __________ stay here as long as you don't interfere.
u  "__________ I help you, madam?"
   "Yes, I'm looking for a yellow pair of shoes."
v  He __________ be very ugly, but he's so sweet and intelligent. I'm in love with him.
w  When I was at school, I __________ do square roots very easily, but I _____(not) now.
x  You _____(not) do that. It's very impolite.
y  You _____(not) take a day off. We have a lot of work at the office these days.
z  You __________ have waited for me. I'm very cross that you didn't.

____________________
1  Pulse aquí para más información sobre verbos defectivos.
2  En este sentido, cabe la posibilidad de usar was/were able to, pero es menos usual.
   As a child, I was able to run and jump all day without getting tired.
3  En pasado, empleamos was/were allowed to para referirnos a una situación concreta o específica. Véase la sección 2 de esta unidad.
   He was allowed to stay up all night yersterday, as it was Christmas Eve. (Se le dejó estar/quedarse levantado toda la noche, ya que era Nochebuena.)
4  Compare esta oración con la siguiente:
   She can't come tomorrow. (No puede venir mañana.)
5  Véase también la unidad 4, sección 7.
6  Had better significa "es mejor que" o "más vale que".