Grammar. Use of the multifunctional Modal verbs "can" and "could". Anatoly A. Yefremov

We know these verbs can have several functions. And I'll try to make them clearer.

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", оr "She can speak fluent Japanese". The negative here is ‘can’t' or cannot….as in: "He can’t swim very well".

In this case, ‘could’ is generally used as the past form of ‘can’. So, “She could speak fluent Japanese when she was young, and she can speak several other languages now too”, оr: “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: «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. ‘Ann could have sent it’ but perhaps ‘Lilly could have written it’…we are not sure who, but I think there are some possibilities.

And now, 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.

Finally, three of the main functions for ‘can/could’ are:
1. to talk about: ability,

2. possibility (with the change in the past form to remember)

3. for requests.


(Edited info from http://blub-club.tu/)

With best regards,

Anatoly A. Yefremov