Can and Could difference

There is a tendency in Indian speech to use "could" for "can", and "would" for "will". This is wrong (or, to avoid being prescriptive, certainly at variance with other varieties of English, and non-standard even in India). Properly, "could" (subjunctive) is used to express possibility, things that may or may not happen, may or may not be done, etc. (to ask polite questions, for instance) — or in the past tense. If you're using it in the same sense as "can" (for a straightforward expression of ability), you're probably using it incorrectly.

For instance:

We could go (if we like / but we won't / etc.), but
We can go (= We are capable of going / It is possible for us to go)
They could see us from the tower (if they were not blind / yesterday), but
They can see us from the tower (=they are capable of seeing us, now)

Wrong: The speaker would be coming tomorrow.
Correct: The speaker will be coming tomorrow. (The progressive is fairly common in IE, but even more natural to just say "The speaker will come tomorrow.")

answered Aug 6 '10 at 4:20

ShreevatsaR

To keep it simple, I answer you without complex grammatical terminology. There are five possible 

situations of using can.

1. Ability

In the first situation, we use can with a meaning of ability. For example, "I think I can lift the box" means that the speaker thinks that she/he is able to lift the box. The past tense form of the sentence is "I thought I could lift the box".

2. Permission

In the second, we use can with the meaning of permission. Undoubtedly, all permissions are questions. Example: "Hey Jim, can I use your PC for awhile?". Use could for more polite forms. Example: "Could you please allow me speak?".

3. Request

In the third case, we use it as a form of request. Example: "Can you please write it for me?". Use could for more polite form. Example: "Could you lend me $100?".

4. Possibility

Sometimes, can is also used to mark a possibility. Example: "Using mobile phones while driving can cause accidents.". Use could if the possibility is uncertain. Example: "He could arrive later."

5. Offer

When offering help to someone, use can. Example: "Can I open the bottle for you?". Could is unusual, formal, and archaic here.

Could is used in two more ways where can isn't normal.

1. To make suggestions

Example: "We could go out for awhile, if you like."

2. To express, forcefully, what someone must do

Example: "You could speak up!"

edited Jun 7 '11 at 2:36 

answered Mar 24 '11 at 7:22