Present ability and possibility
We use can and can’t to talk about a present or general ability or skill (or lack of ability). The meaning is something like
‘know how to do something’.
This is especially common with the verbs ‘speak’ or ‘play’ when we’re talking about languages, instruments, games and sports.
• She can speak Italian.
• He can play the piano.
• She can’t speak English.
We can also use can to talk about something that we succeed in doing in the present moment.
• Look – I can lift the wardrobe!
We can also use ‘can’ with a specific possibility meaning. Here ‘can’ doesn’t mean that you know how to do something, but rather that circumstances let you do something or something is possible because of something external.
• I can swim. (= I know how to swim.)
• I can pick up the children tomorrow. (= Circumstances let me pick up the children / it’s possible for me to pick up the children because there is a train at the right time and my meeting will be finished.)
We often use this meaning of ‘can’ with inanimate objects. A feature of the object makes something possible.
• The car can go 150km per hour.
• This room can hold two hundred people.
Future ability and possibility
We don’t usually use ‘can’ for ability in the future when we’re talking about skills or things we’re learning how to do.
Instead, we use ‘will be able to’. An exception is in subordinate clauses where we can use the present simple with a future meaning – in this kind of clause, we can also use ‘can’. (See the lesson about the present simple in Terrific Tenses.)
• I’ll be able to speak French after my French course.
• NOT: I can speak French after my French course.
But it’s okay to say.
• I’ll come to visit you in Paris when I can speak French.
When we’re talking about possibility (being able to do something because of circumstances), we can use ‘can’ or ‘will be able to’.
We use ‘could’ and ‘couldn’t’ in the past to talk about general ability (= know how to do something / have the skill to do something). We can also use ‘was / were able to’.
• I could read when I was six.
• I couldn’t read when I was two.
• He could always persuade his mother to give him money.
However, when we’re talking about ability on one specific occasion in the past, then we don’t use could. Instead, we need to use ‘managed to’ or ‘was / were able to’.
• I looked everywhere but I couldn’t find my keys.
• NOT: I looked everywhere and I could find my keys.
• I looked everywhere and I managed to find my keys.
• I looked everywhere and I was able to find my keys.
This is the same with possibility (being able to do something because of circumstances). We don’t use ‘could’ for one
However, if there is a negative adverb or a question, or if we succeed only with a lot of difficulty (often with words like
‘only’ or ‘just’), we can use could for a specific past ability.
• I was so tired I could hardly finish the race. • Could you move the wardrobe by yourself?
• I could just reach the top shelf.
• She could only find one sock.
We can also use ‘could’ for specific past ability / possibility in some subordinate clauses.
• I’m really pleased that you could make it.
Conditional and hypothetical ability and possibility
We can use ‘could’ to talk about present or future ability if we are using an unreal conditional or talking about something that’s hypothetical.
• You could pass the exam if you just stayed at home and studied.
• With enough time, I could do it myself. (But I don’t have enough time.)
It’s the same for things that are hypothetically possible.
• I could meet you at eleven, but I’d be really tired the next day.
This also works for things that are impossible because we really, really don’t want to do them.
• I couldn’t touch a snake!
Unrealised past ability or possibility
In a similar way to other modal verbs with ‘have + past participle’, we can use ‘could have + past participle’ to talk about a past ability or possibility that we didn’t use.
• I could have gone to art school, but my parents didn’t want me to. (= I would have been able to go, but I didn’t go.)
• I could have gone to the gym yesterday, but I was feeling lazy. (= It was possible for me to go, but I didn’t go.)
This structure can also be used to criticise people. It means something like ‘you had the ability or possibility to do something, but you didn’t do it and that wasn’t right’.
• You could have called me to say you’d be late! I was worried. (= You were able to call me but you didn’t. You should have called me.)
Emphasise effort or feelings in the past
We can use ‘couldn’t + have + past participle’ with a comparative to show that some action was the most possible. This is usually used to show something is very true.
• I couldn’t have been more pleased! (= I was the most pleased that it’s possible for me to be = I was very, very pleased.)
• She couldn’t have worked harder. (= It was impossible for her to work any harder = she worked very, very hard.)
• She couldn’t have been more helpful. (= She was very, very helpful.)
General or typical possibility
We also use ‘can’ to say that something often happens or is possible in a certain situation. It’s similar to talking about a typical characteristic of something.
• London can be cold in June.
This means that ‘sometimes London is cold in June’ or ‘it’s possible for London to be cold in June’.
We use ‘can’ to talk about theoretical or general possibility, not probability (the chance of something happening). For probability, we use ‘may / might / could’.
• The train might be late today. (= There’s a chance that the train will be late today.)
• NOT: The train can be late today.
• Trains can be late. (= Sometimes trains are late.)
We use ‘could’ for the past of this use of ‘can’.
• My village had only one bus an hour and the bus could be very late.
• It could snow in June in Scotland 100 years ago.
Perception verbs
‘Can’ is often used with certain verbs: see / hear / smell / feel / remember / guess / tell (when it means ‘understand’).
When talking about perception – when we don’t try to do the action but it just happens automatically - these verbs aren’t usually used in the continuous. Instead, we often use them with ‘can’, especially in UK English.
• I can smell smoke.
• I can’t see our house – am I looking in the right direction?
• I can’t remember.
This is different from the ability meaning of ‘can’. ‘I can smell’ often doesn’t mean that the person has the ability to smell (though we can also use ‘can’ in that way ‘I can’t smell – I’ve got a horrible cold’). Instead, we usually use ‘can’ in this way when we are talking about something that’s happening at this moment. It means that it’s possible for me to smell something particular because of the circumstances. And it’s used in a similar way to the present continuous, to emphasise that something is happening now.
With perception verbs in the past, we use ‘could’ and ‘couldn’t’ (even for one specific instance).
• I could smell smoke, so I called the fire brigade.
• She couldn’t see the house form the top of the mountain, because it was foggy.
There’s not a huge difference in meaning between using ‘can / could’ here and using a normal tense of these verbs. But it’s more natural to use ‘can / can’t / could / couldn’t’, especially with see / hear / smell / taste.