The use of ‘can/could’ and ‘to be able'

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 adjective ‘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.

More about "able" and "can"


Difference between can and be able to
(https://www.englishgrammar.org/difference-3/)

Both can and be able to can be used to talk about ability. In some cases they are interchangeable.

Using "can": Can is used in the present tense. It is used to talk about our ability to do things.
Examples

Be able to is also possible in these cases; however, it sounds a bit more formal here.

I am able to knit. (Less natural than ‘I can knit.’)

She is able to speak English well.

To talk about our past ability, we use could. Was/were able to is also possible. Study the examples given below.

She could read when she was three.

OR She was able to read when she was three.

Again, in spoken English, we are less likely to use was able to. As you can see (not As you are able to see) in all of these sentences, we were talking about general ability. Things that we can or we could do at any time in the present or in the past.

To talk about things that we managed to do on specific occasions in the past, we cannot use could. Instead, we use was/were able to. The verb managed, succeeded (in…ing) are also possible in this case.

I was able to get some really good bargains in the sale. (NOT I could get some really good bargains in the sale.)

After climbing for several hours, we managed to get to the top of the mountain.

OR After climbing for several hours, we were able to get to the top of the mountain. (NOT … we could get to the top of the mountain.)

Both could and be able to can be used to say that we were not capable of doing something on a specific occasion.

In spite of climbing for hours, we couldn’t get to the top of the mountain. OR In spite of climbing for hours, we weren’t able to get to the top of the mountain.

~~~~~~~~~~~~~~