Ability, dynamic possibility, permission, propensity
Can (negative can’t, cannot) is the modal verb that expresses these three types of dynamic modality:
Ability
Can you reach the top shelf? (Are you able to reach . . . ?)
Ability includes mental and acquired accomplishments:
Can you read Arabic? (Are you able /Do you know how to read Arabic?)
If you can’t swim, stay close to the shore. (If you are not able to swim . . . )
Possibility
This paint can be applied with a spray. (It is possible to apply this paint . . . /for this paint to be applied) (dynamic possibility)
It is important to distinguish dynamic possibility, which is glossed as ‘It is possible to . . . /for’ from epistemic (predictive) possibility, which is glossed as ‘It is possible that . . . ’.
Can is not used for the expression of epistemic possibility ( *It can be true) except as the negation of must (It can’t be true). Compare:
I can be there by 10. (= It is possible for me to be there by 10) (dynamic possibility)
I may /might be there by 10. (= It is possible that I may /might be there by 10) (epistemic
possibility)
I could be there by 10. (either predictive or dynamic meanings, or both)
Permission: can, may
May is a more formal alternative to can in the meanings of permission , and is extended to such meanings as polite offer:
May I come in? Yes, you may. (permission to enter requested and granted)
May I help you with the luggage? (polite offer)
Permission in a past event is expressed by be allowed to: They were allowed to go.
Can is far less formal than may, and has supplanted may except in the most formal registers.
Can we smoke in here? I’m afraid not. You may not /can’t smoke.
Can I borrow your car? Oh, all right, but be careful.
Might is sometimes used for an indirect request:
You might fetch me a coke and a packet of crisps.
Can is used with verbs of perception such as see, hear, feel and taste, and with
cognitive verbs such as imagine, guess, grasp and understand:
I can’t hear anything with all this noise. *I don’t hear
Can you see the time on Big Ben over there?
I can taste the pepper in this soup.
Can you smell gas?
I can feel the spray from the waterfall on my face.
You can imagine how we felt.
The students couldn’t follow his train of thought.
Could /was able as the past of can
Dynamic could is used as the past of can only to express an action or state extended indefinitely, that is, imperfective aspect. Was/were able is used for single, holistic achievements:
You could hear the sound of the rocket in the air.
He was able to escape in time. (not *he could escape)
This distinction is obligatory only in the affirmative and interrogative. In the negative, couldn’t and be able are interpreted as having the same result and are therefore interchangeable.
He wasn’t able to escape; he couldn’t escape.
Since can has no infinitive, be able is used whenever an infinitive is required, as in:
She would like to be able to dive.
I would have helped you if I had been able to.
