The Grammaring Guide to English Grammar with Exercises

Modals to express ability: CAN, COULD, BE ABLE TO

Present and future

Can and be able to can express present and future ability:

Tom

can play

the piano.

We

can't enter

this street because it's blocked.

I

am not able to help

you at the moment.

If you ask him, he

can

probably

help

you.

I

will

probably

be able to get

there by 9.

Past

Could and was/were able to can express general past ability:

I

could swim

when I was five.

I

was able to swim

when I was five.

But the past ability to perform a particular action on one occasion is expressed with was/were able to:

Although the current was strong, I

was able to swim

to the other bank.

(I managed to swim to the other bank)

In the sentence above could is not possible. However, this rule is relaxed in the negative and with verbs of the senses:

I read it but I

couldn't understand / wasn't able to understand

it.

I

could see / was able to see

him through the window.

I

couldn't walk / wasn't able to walk

any further.

Be able to is used to express past ability when could is grammatically not possible:

I

have been able to swim

since I was five.

Quotes:

Not everything that can be counted counts, and not everything that counts can be counted. - Albert Einstein

If one cannot invent a really convincing lie, it is often better to stick to the truth. - Angela Thirkell

It is the mark of an educated mind to be able to entertain a thought without accepting it. - Aristotle

No one can make you feel inferior without your consent. - Eleanor Roosevelt

If you can find a path with no obstacles, it probably doesn't lead anywhere. - Frank A. Clark

I have never met a man so ignorant that I couldn't learn something from him. - Galileo Galilei

There is no man living that cannot do more than he thinks he can. - Henry Ford

A coach is someone who can give correction without causing resentment. - John Wooden

It is the friends you can call up at 4 a.m. that matter. - Marlene Dietrich

If you don't like something change it; if you can't change it, change the way you think about it. - Mary Engelbreit

No river can return to its source, yet all rivers must have a beginning. - Native American proverb

You must learn from the mistakes of others. You can't possibly live long enough to make them all yourself. - Samuel Levenson

Action will remove the doubts that theory cannot solve. - Tehyi Hsieh

Only in grammar can you be more than perfect. - William Safire