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'Can' expresses ability in the present; 'be able to' is the necessary alternative when 'can' cannot be used.
Side-by-side comparison
"Can" expresses ability or possibility in the present. The problem: it has no future, perfect, or infinitive forms of its own. For those cases use "be able to".
She can run five kilometres without stopping, she trains every morning.
Can you hear that noise? I think something is wrong with the engine.
I can speak English, but I cannot write it as well as I would like.
"Be able to" covers the tenses where "can" does not reach: future, perfect, infinitive. It also emphasizes that something was achieved on a specific occasion, which "can" does not.
He was not able to sleep at all the night before the exam.
After weeks of practice she was finally able to play the song from memory.
Will you be able to join us for dinner on Friday evening?
Common mistake
- •"I will can attend the meeting" is wrong: "can" cannot follow another auxiliary.
- •In future, perfect, or after another modal, use "be able to": "I will be able to attend".
- •And for a specific positive past achievement, use "was able to", not "could": "Yesterday she was able to finish the report on time".
Trick to remember it
Is there an auxiliary or compound tense before it? Use "be able to", never "can". "Will be able to" (future). "Have been able to" (perfect). "Might be able to" (modal). "To be able to" (infinitive).
Quick rule
- •"Can" = ability in present and past (could).
- •Not valid in future, perfect, or after another modal.
- •"Be able to" = ability in any tense and form.
Did you know?
"Can" is one of the oldest verbs in English, its Germanic root "kunnan" meant "to know how to do something". "Can" is a defective modal: it has no infinitive, no gerund, no compound forms. This is exactly why we need "be able to" for all the grammatical contexts where "can" cannot appear.
Practise what you learned
Exercise 1 · Pick the right one
She _____ speak three languages. She grew up in a bilingual family.
Exercise 2 · Pick the right one
After months of practice, she _____ finally play the song perfectly.
Exercise 3 · Form the word
Related word pairs
'Could you...?' asks if someone is able to do something; 'would you...?' asks if they would be willing to.
'Must' indicates obligation (you have to do it); 'mustn't' indicates prohibition (you must not do it).
'A' is used before consonant sounds; 'an' is used before vowel sounds.
'A lot of' works for countable and uncountable; 'many' only for countable; 'much' only for uncountable.
Bryan López
Founder of SpeakUP Academy