Modal Verbs (I)

Main Characteristics

Modals are special verbs which behave very irregularly in English. These verbs have common features, which are:

1) They don't need auxiliary verbs for questions and negatives (except 'have to'):
     I can't go to the party.
     Should I give up smoking?
     You don't have to come tomorrow. (exception)

2) They go with a bare infinitive, that is, an infinitive without 'to' (except 'have to' and 'ought to'):
      They could play rugby.
      He might come home next weekend.
      She ought to use a seat belt when she drives. (exception)

3) They don't agree in the 3rd person singular present simple:
     She can play the violin beautifully
     She cans play the flute. (error!)


Expressing Ability and Permission

One possibility of expressing ability and permission is by means of the modal auxiliary verbs can and could, the first one in the present simple and the second one in the past simple and conditional. For other tenses, we need the different forms of be able to for ability and be allowed to for permission. Look at these examples below.

Ability:
When I was eight, I could speak good English.
They can play the piano and the keyboard.
Jennifer will be able to pass the year.
I've never been able to ride a motorbike.

Permission:
We couldn't be late on Saturday nights when we were teenagers.
You can go to the toilet now.
She will be allowed to enter without a ticket.
The children haven't been allowed to play in the gardens.

When asking for permission, leaving aside the differences in use of present and past time, we must also have in mind politeness or courtesy, in other words, formality, where can is less polite than could.
Could I borrow your pen, please?
Can I ask you a question?

May is possible as well when expressing permission, just like could but may is even more formal and polite, and especially used in requests:
May I see your identity card, Sir?
You may leave now if you wish.

Don't forget a thing: when translating can/could, if it expresses permission, just use the forms of 'poder' but if it conveys ability then use the paradigm of 'saber.'

Practice:
Exercise1, Exercise2, Exercise3.