Can and be able to are both used to talk about ability. Sometimes they’re interchangeable, but sometimes they’re not. So, let’s start by looking at your examples. You’ve asked about can’t and unable to. Well, we use can’t in the present tense when we say that we don’t have the ability to do something — for example, I can’t swim.
We could also say I am unable to swim, and the meaning would be pretty much the same. But which one do we choose? Well, for informal English, and for spoken English in particular, people tend to use can or can’t. Able to seems a bit more formal and not really appropriate for everyday situations.
Now the past of can is could, and the past of be able to is was able to or were able to. For example, we can say:
I could swim (or I couldn’t swim ) when I was 7 years old
or we can say
I was able to swim (or I was unable to swim ) when I was 7 years old
Again, in spoken English, we’d probably use could or couldn’t to talk about past ability or lack of ability. Now the examples I’ve just given are concerned with general ability.
I could swim when I was 5 years old
It refers to a general ability, not an ability in a specific past incident. It’s important to understand this difference because when we talk about ability in specific past incidents, the way we use could and was able to changes. I’ll tell you a story as an example.
When I was 10 years old, my mother and father took me on a sailing holiday, and on the last day, when the sea was very rough, I fell out of the boat! It was OK though, I was able to swim back to the boat.
I was able to swim back to the boat.
It’s describing an ability in a specific past event, and we would NOT normally say
I could swim back to the boat.
But what about the negative form? Well, on the same trip, my younger sister fell into the water too. But she wasn’t a strong swimmer, and my father had to jump into the sea and rescue her, because she couldn’t swim back to the boat. So to talk about lack of ability in a specific past event, couldn’t is OK. We could also say:
She wasn’t able to swim back to the boat or
She was unable to swim back to the boat.
OK? Now I’m going to deal with future, present perfect and past perfect tenses, by telling you that can is not normally used to describe ability in these tenses. For these tenses, you have to use be able to or be unable to. Here are some examples and don’t forget that when you use unable to, it gives a more formal tone than not able to. Here’s an example in the future:
You’ll be able to speak really good English by the end of the year. And now one in present perfect:
We’ve been unable to complete the project within the specified budget.
The next one is present perfect, question form:
How long has your little boy been able to read?
And let’s listen to a past perfect — this one’s negative:
He was given a detention because he hadn’t been able to finish his homework.
Now, although we’ve just said that we don’t usually use can or can’t to talk about future ability, it is possible to use can to suggest a possible future action, like this:
I can join you at the meeting, but I can’t stay very long.
|Positive||Negative structure / meaning||Question|
|Present||I can workI am able to work||I can’t workI am not able to work
I am unable to work
|Can I work?Am I (un)able to work?|
|Past||I could workI was able to work||I couldn’t workI was not able to work
I was unable to work
|Could I work?Was I (un)able to work?|
|Future||I will / shall be able to work||I won’t / shan’t be able to workI will / shall be unable to work||Will / shall I be (un)able to work?|
|Present perfect||I have been able to work||I haven’t been able to workI have been unable to work||Have I been (un)able to work?|
|Past perfect||I had been able to work||I hadn’t been able to workI had been unable to work||Had I been (un)able to work?|