A question from Afreen in India :
what is the difference between - 'can' and 'could.'
Sian Harris answers:
This is a common area for questions, as these verbs can have several functions. Without going into too much detail, I'm going to try and illustrate the key uses for you.
Firstly, we use 'can' for something that is possible or to show that somebody has the ability to do something in the present and future. For example:
We can see the park from our house,
Ella can speak fluent Japanese.
The negative here is 'can't'...as in: He can't swim very well.
In this case, 'could' is generally used as the past form of 'can'. So,
Ella could speak fluent Japanese when she was young, and she can speak several other languages now too.
When I was a child I could run fast.
We use 'could' for general past ability, but be a little bit careful here, because when we're talking about what happened in a particular situation, we tend to use 'was/were able to' for past ability instead:
For example: The fire spread quickly, but luckily everybody was able to escape.
We also use 'could' to talk about possible actions now and in the future - here the function is possibility (not ability) - and this is what tends to cause the confusion around these words. For example, if you are expecting some friends to visit, but they have been delayed, you might say 'they could arrive at any time now', or if you're trying to make progress with your work you might ask 'Could we talk to the boss again? For this function of possibility, we need to watch out for the past form, as this doesn't work in exactly the same way. If we want to express past possibility, we need to use could + have and the past participle (have done, have been etc). Let's imagine you've received a letter or card from a mystery admirer, and you're trying to work out who sent it to you. As the sending happened in the past, we are speculating about a past possibility. 'John could have sent it' but perhaps 'James could have written it'...we are not sure who, but think there are some possibilities.
Finally, let's look at how we commonly use can/could in question forms to make requests (or ask for things). If we go shopping for clothes, we might ask 'can/could I try that dress on please?' or 'could I see those shoes in blue.' Here, either 'can' or 'could' may be used without significant difference. The only thing worth noting is that in terms of register, that is (style and formality), 'could' is considered slightly more formal or polite.
So, to recap, three of the main functions for 'can/could' are to talk about: ability, possibility (with the change in the past form to remember) and for requests.
Sian Harris is the Manager of English Language Training & Development at the BBC World Service, and runs specialist courses in London and overseas for BBC staff. Before joining the BBC, she spent 10 years as an English language teacher, examiner and academic manager in schools and colleges in London.
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