136 can and be able: forms
can is used here in conjunction with be + the adjective able, which supplies the missing parts of can and provides an alternative form for the present and past tense. We have therefore the following forms:
Infinitive: to be able Past participle: been able
Affirmative
Negative
Interrogative
Future
will/shall be able
will/shall not be able
shall/will I be able? will he be able? etc.
Present
can or
am able
cannot or am not able
can I? or am I able? etc.
Past
could or was able
could not or was not able
could I? or was I able? etc.
There is only one future form, for can is not used in the future except to express permission. In the conditional, however, we have two forms: could and would be able.
All other tenses are formed with be able according to the rules for
ordinary verbs:
Present perfect: have been able
Past perfect: had been able
Negative interrogative: could you not/couldn’t you? were you not/weren’t
you able? will you not/won’t you be able? etc.
can/be/will/shall not and have can be contracted in the usual way:
/ wasn ’t able, he won’t be able, I’ve been able.
can is followed by the bare infinitive.
be able is followed by the full infinitive.
137 can/an able, could/was able
A can and be able
1 shall/will be able is the only future form:
Our baby will be able to walk in a few weeks.
13 Ability
2 Either can or am able may be used in the present, can is the more usual:
Can you/An you able to type?
I can’t pay you today. Can you wait till tomorrow? or
Could you wait? (request; see B2 below)
3 For the present perfect, however, we must use the be able form:
Since his accident he hasn’t been able to leave the house.
B could
1 could can be used with a present meaning when there is an idea of condition:
Could you run the business by yourself? (if this was necessary) Could he get another job? (if he left this one) / could get you a copy, (if you want one) In the first two examples could is replaceable by would be able.
2 could you? is a very good way of introducing a request. It is an alternative to would you? and a little more polite:
Could you show me the way/lend me £5/wait halfanluntr? Could you please send me an application form? couldn’t you? is also useful:
HOUSEHOLDER: Could you come and mend a leak in a pipe? PLUMBER: Would sometime next mmth suit you? HOUSEHOLDER: Couldn’t $ou come a little earlier?
C could and was able used for past ability
1 For ability only, either can be used:
When I was young I could/was able to climb any tree in the ’forest.
2 For ability + particular actio’n, use was able:
Although the pilot was badly hurt he was able to explain what had
happened. (He could and did explain.)
The boat capsized quite near the bank so the children were able to
swim to safety. (They could and did swim.)
This rule, however, is relaxed in the negative when the action did not take place, and with verbs of the senses:
He read the message but he couldn ’t/wasn ’t able to understand it.
I could/was able to see him through the window.
D had been able is the past perfect form:
He said he had lost his passport and hadn ’t been able to leave the country.
(For could in reported speech, see 312.)
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134
135
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13 Ability
could + perfect infinitive
This form is used for past ability when the action was not performed:
/ could have lent you the money. Whydidn ’(you ask me?
(see also 154) or when we don’t know whether it was performed or not:
The money has disappeared! Who could have taken it?
Tom could have (taken it); he was here alone yesterday. Compare:
He was able to send a message. (He sent it.)
He could have sent a message. (He didn’t send it or we don’t know
whether he sent it or not. See also 135.)
could + perfect infinitive can also express irritation at or reproach for the non-performance of an action:
You could have told me =
/ am annoyed/disappointed that you didn ’t tell me. You should have
told me.
There would be a strong stress on the word the speaker wishes to emphasize. (For might used in the same way, see 285.)
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