Can, could, be able to and be allowed to

CAN, COULD, BE ABLE TO and BE ALLOWED TO (MODALS OF ABILITY)

We sometimes use BE ABLE TO instead of CAN and COULD to talk about ability. However, we avoid BE ABLE TO –

When we talk about something that is happening as we speak:

Look! I can hold this bar only using a hand. 

Passive:

All recordings can now be downloaded automatically. 

When the meaning is ‘know how to’: 

Can you speak English well? 

If we talk about a single achievement, rather than a general ability in the past, we usually use BE ABLE TO rather than COULD. Compare: 

When I was a child, I could swim fast. (General Ability in the past) 

I was able to cross the river easily. (A specific achievement) 

However, COULD is usually more natural — 

In Negative Sentences:

I tried to wake up early but I couldn’t hear my alarm. (rather than… I wasn’t able to hear my alarm.) 

With verbs of the senses – feel, hear, see, smell, taste — and with verbs of ‘thinking’ (For example: believe, decide , remember, understand)

I could smell what he was cooking in the kitchen. 

To suggest that something didn’t happen, particularly with almost, hardly, just, nearly: 

I could nearly burn the pancakes. (rather than I was nearly able to…) 

CAN, COULD, BE ABLE TO and BE ALLOWED TO (MODALS OF POSSIBILITY) 

To talk about the theoretical possibility of something happening we use COULD, not can. However, we use CAN, not COULD, to say that something is possible and actually happens. 

Compare:

It could be expensive to have a dog. (= if we had one, it could or it may not be expensive) 

It can be expensive to have a dog. (= it can be, and it sometimes is) 

We use CAN’T, not couldn’t, to say that something is theoretically or actually impossible: 

The dentist can’t see you this afternoon. He is out of the clinic. 

There can’t be many people in the world who don’t eat spicy food. 

We use CAN to indicate that there is a very real possibility of a future event happening. Using COULD suggest that something is less likely or that there is some doubt about it. 

Compare:

We can buy some strawberries when we visit him. (= we will be able to buy some strawberries) 

We could buy some strawberries when we visit him.  (= it is possible; if it is available there.) 

COULD and BE ALLOWED TO (MODALS OF PERMISSION)

To say that in the past someone had general permission to do something — that is, to do it at any time – we can use either could or was/were allowed to. However, to talk about permission for one particular past action, we use was/were allowed to, but not could. 

Compare:

Anyone was allowed to pick some oranges when the owner was there. (or … could pick…) 

Although he didn’t pay his tuition fee, he was allowed to take the exam. (not… could take.)