Can/could vs. to be able to

The use of ‘can/could’ and ‘to be able to’ in order to express ability, can at times cause confusion. So, today we are going to look at these different verb constructions and how they are used the same, and differently, to talk about ability. Here we go.

Can and to be able to are usually used in the same way without a difference in meaning, but not always…

‘Can’ is a modal auxiliary verb that expresses general ability in the present tense; ‘could’ works the same way, but in the past tense.

‘To be able to’ is not a modal auxiliary verb. It is the conjugated verb ‘to be’ + the adverb ‘able’ + the infinitive ‘to’.

When talking/writing about ability in the present tense you can use either ‘can’ or ‘be able to’, but ‘can’ is generally more common among native speakers.

Examples:
I can speak two languages.
I am able to speak two languages.

When talking/writing about past ability you can use ‘could’ or ‘was/were able to’ in order to discuss an ability that existed for a long time, but now no longer exists.

Examples:
When I was young, I was able to speak two languages.
When I was young, I could speak two languages.

But you should only use ‘was/were able to’ with action verbs to talk about an ability related to a single event or incident in the past.

Examples:
I was able to surprise my teacher yesterday.
Don’t say: I could surprise my teacher yesterday.

You can use either ‘could’ or ‘was/were able to’ with some stative verbs (see, hear, feel, taste) to discuss ability related to a single event or incident in the past.

Examples:
I was able to see the teacher well from my seat yesterday.
I could see the teacher well from my seat yesterday.

It is also okay to use ‘couldn’t’ or ‘wasn’t/weren’t able to’ for negative statements for both single events and a long period of time in the past.

Examples:
I wasn’t able to surprise my teacher yesterday. or I was never able to surprise my teacher.
I couldn’t surprise my teacher yesterday. or I couldn’t ever surprise my teacher.

Lastly, when talking/writing about future ability in English we only use ‘will be able to.’ We never use ‘can’ for an ability that may exist in the future.

Examples:
When I finish the advanced level English class I will be able to speak like a native speaker.
Don’t say: When I finish the advanced level English class I can speak like a native speaker.

But it is okay to use either ‘can’ or ‘will be able to’ when we are talking about decisions and future arrangements.

Examples:
I can attend the class next week.
I will be able to attend the class next week.

I hope this post helps clear up any confusion you had about the different uses of ‘can/could’ and ‘to be able to’ when you are talking about present, past, and future ability.