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// On :Minggu, 27 Oktober 2013
Expressing Ability with Modal Verbs Can, Could,
Be Able To
Improve your knowledge of English
modal verbs and learn how to express ability with 'can', 'could' and 'be able
to'.
I sometimes think that God, in
creating man, overestimated His ability, once humorously remarked Oscar Wilde,
a famous Irish writer and poet of the 19th-century. What exactly is ability? It
is the physical or mental skill needed to do or create something. In English
grammar, ability is usually expressed with the modal verbs can or could, and
with the semi-modal construction be able to. The English language distinguishes
between general ability and ability used in particular situations.
Can
The
modal verb can is used to talk about present and future abiliy. The negative of
can is cannot (contraction: can't), which expresses lack of ability to do or
create something.
- I can dance. / Can you drive? / Can you speak any foreign languages? / I can't sing. / I can visit you tomorrow.
You can also use the construction be
able to to talk about present and future ability because it is interchangeable
with the modal verb can. However, can is much more commonly used while be able
to is more formal and less commonly used.
- Are you able to swim?
The negative of be able to is be not
able to.
- I'm afraid I'm not able to fulfil your request.
Could
and Was / Were Able To
When expressing ability, could can
be used as the past of can. The modal verb could (used for past ability) is
used for general abilityto say that somebody had the ability to do or
create something at any time in the past, whenever he or she wanted.
- When I was younger, I could run 20km without stopping.
- When I was younger, I could run faster than anyone else.
The construction was / were able to
is also possible here. For example:
- Were you able to read before you started school?
- When I was younger, I was able to run faster than anyone else.
In negative sentences that express
lack of general ability, we can use either couldn't or wasn't / weren't able
to.
- I couldn't swim when I was younger.
- I wasn't able to drive until I was thirty-six.
When we want to say that someone had
the ability to do something in a particular past situation, we must use
the construction was / were able to (could is not normally used to say that
somebody did something on one occasion in the past).
- Even though I had hurt my leg, I was able to walk back to the house.
- Were you able to do your homework last night?
Compare:
General ability
- Danielle was an excellent table tennis player. She could beat anybody. (=In the past, she had the general ability to beat anybody)
Particular situation
- Daniel and Danielle had a game of table tennis last week. Daniel played very well but in the end Danielle was able to beat him. (=Last week, Danielle managed to beat Daniel in that particular game.)
In negative sentences that express
lack of ability in particular past situations both couldn't and wasn't /
weren't able to are possible.
- Yesterday I wanted to buy some weed killer but couldn't find any.
- Yesterday I wanted to buy some weed killer but wasn't able to find any.
WARNING: There is an exception with the
verbs of sense perception (see, hear, smell, taste,
feel) and some verbs of thinking (understand, remember,
decide, believe, guess) When we form sentences that
express ability in particular past situations, we must always use the
modals could or couldn't. Was / were able to or wasn't / weren't able to are
never used with these verbs. For example:
- When I came into the kitchen, I could smell something burning.
- When I looked out of the window, I could see a man walking down the street.
- The woman spoke only French, therefore I couldn't understand what she said.
- I couldn't decide whether to ask her out or not.
Please note that there is no extra
explanation for all these rules. Just try to commit them to memory.
Could
Have + Past Participle
English also has a special structure
could have + past participle that is used to talk about unrealised past
ability or opportunity. This structure is used when we want to say that
somebody had an ability or opportunity to do something in the past, but did not
do it. For instance:
- A: You could've married Sharon Stone! Why didn't you? B: Dunno. I guess I didn't find her attractive enough.
- A: You could've rescused that child from drowning! You're an excellent swimmer! Why didn't you?! B: I was afraid to risk my own life. There are crocodiles in the river!
The
Semi Modal Verb Be Able To
There are certain things you should
know about the semi-modal verb be able to. First of all, let me explain why is
it called a semi-modal verb. Be able to falls into the category of semi-modal
verbs because it is modal in meaning but not in form. What does this
mean? Pure modal verbs have no tense forms. Be able to, on the other hand, is
different. It can be used in the full range of tense forms, therefore it is not
a pure modal verb. Now let's return to the modal verb can for a moment. Can has
no infinitive, no ing form, no perfect form and no
future form. Therefore, it needs some assistance from its semi-modal
sibling be able to. Let me illustrate:
- I would like to be able to play the piano well. (We cannot say I would like to can play the piano.)
As you can see, the modal verb can
has no infinitive form.
- I enjoy being able to speak five foreign languages. (We cannot say I enjoy canning speak five foreign languages.)
As you can see, the modal verb can
has no ing form.
- My husband hasn't been able to walk since his car accident. (We cannot say My husband hasn't could walk since his car accident.)
As you can see, the modal verb can
has no perfect form.
- In the future, people will be able to live on Mars. (We cannot say In the future, people will can live on Mars.)
As you can see, the modal verb can
has no future form.
We must also use be able to (not
can) in sentences that already contain a modal verb. For example:
- I might (modal verb) be able to help you. (We cannot say I might can help you.)
Modal verbs belong to the category
of auxiliary (helping) verbs. We do not use them on their own but instead have
to use them in conjunction with another (main) verb. Modal verbs express
ability, possibility, necessity, obligation, etc.
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