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When do I use “can” or “could”?

I wonder which of the following is more polite:

  1. Can you please change my email address?
  2. Could you please change my email address?

The second one sounds to me as if the request is more urgent. So I would choose the first. But I heard that "could", "should", etc. forms are more polite forms of "can", "shall", etc. Is this true at all and if so, what is the underlying reason?

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marked as duplicate by FumbleFingers, Robusto, MετάEd, waiwai933 Jan 25 '13 at 3:29

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Politeness isn't the issue. They mean slightly different things in fact. – Noldorin Dec 23 '10 at 22:28
May I suggest another word? – Mateen Ulhaq Dec 23 '10 at 22:40
@Noldorin: yes, they mean slightly different things, but one of the meanings is slightly more polite than the other. Hence, it is an issue of politeness. – Marthaª Dec 23 '10 at 23:53
@Martha: Well yes, it's become an issue of politeness but in actuality it's just grammatical/meaning correctness. When you're being polite, you're really being grammatically correct! – Noldorin Dec 24 '10 at 1:45
up vote 10 down vote accepted

The problem is that "can you x" is like asking "are you able to x" whereas "could you x" is like asking "are you willing to x"

So yeah, politeness isn't the issue at all.

This is probably why, even if you're not aware of the nuanced meaning, could is more polite. It's not insulting the target by questioning their ability to perform something.

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Taking that thought further 'could' implies 'Can you you this for me at this point, with consideration for the interruption it might cause. While it's a stretch, it could even be interpreted explicitly submissive as in 'I know it's only me, but could you do this for me too?' – Joost Schuur Dec 23 '10 at 23:49
This answer seems to be contradicting itself: it says "politeness isn't the issue", and then goes on to detail exactly why politeness IS the issue. //confused – Marthaª Dec 23 '10 at 23:55
Martha it's only a politeness issue if you assume the speaker didn't realize there was a nuance in meaning. – Swizec Teller Dec 24 '10 at 0:47
-1: 8 upvotes, and this answer has the meaning of could confused with would? – Glen Wheeler Jun 24 '11 at 6:15
@SwizecTeller, Context is important. Depending on what x is, they could very well mean the same thing. In fact, with context, the reader would often not misread what the sentence means, regardless of whether "can" or "could" had been used. – Pacerier Jun 3 at 5:04

I stumbled on this old question and can't help but think the answers don't quite get to the heart of the matter, so here goes.

To answer the question: "could" definitely sounds slightly more polite than "can" to a native speaker since it is less direct and more deferential as a result.

"Could" is a form of "can", so both are technically asking "are you able to...". This is not the difference between the two.

The difference is that "could" is used with the conditional mood in English. There is an unstated by implied "if" clause here. The sense of the sentence is, "if you weren't too busy at the moment, could you X?" This is why it is less direct and more deferential.

"Could" does not have the sense of "are you willing to X?". That is "would". I could also ask "would you X?" This is polite, don't worry about it in practical terms. If I had to think very hard about it, I'd say "would" is slightly more direct and assertive. It is questioning whether the person wants to do X. "Could" asks whether they can do X. And that is maybe more indirect. It turns the question away from "do you want to do this for me" to "are you in a position to do this for me at all?"

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I wonder what [WHICH] of the following is more polite:

Can you please change my email address? Could you please change my email address? The second one sounds as if the request is more urgent, to me. So I would take the first. But I heard the "could", "should", etc. forms are more polite forms of "can", "shall", etc. What's the underlying reason and is this true at all?

The underlying reason for the differing levels of politeness in modal use is how they developed historically.

Hundreds of years ago, modal verbs had tense. There were present tense forms and there were past tense forms. Such is no longer the case. In modern English, modal verbs are tenseless. This allows them to operate in all time situations, past, present and future.

This does not mean that each and every modal verb can do each and every modal function. They are still somewhat constrained by their historical roots and the meanings they hold today.

Historical present tense form - Historical past tense form

can - could

will - would

may - might

shall - should

In English, the past tense FORM of both lexical verbs [jump, run, write, etc] and the Historical past tense FORM of modal verbs are used to perform a number of different tasks: politeness/deference/doubt/non- real or counterfactual/... .

"I was wondering ..." is more polite/deferentialsofter than "I'm wondering ... "

"Did you want something to eat?" is more polite/deferential/softer than "Do you want something to eat?"

It's vitally important to remember that this use is only a use of the past tense FORM, it is NOT a true past tense, in the sense of time, use!!

Another important thing to remember is that when it comes to the modals, it is the HISTORICAL past tense FORMS which are used, again, to effect a greater sense of politeness, deference/softness/... .

Even the Historical present tense FORMS are polite but they are not as polite as the HISTORICAL past tense FORMS because they are part of this grammatical "English uses past tense FORMS to effect greater politeness/etc" routine.

In your examples, there is no difference in meaning between 'can' and 'could'. In this sense both hold the meaning of "Is it possible for you to change my email address?"

There is a difference in the level of politeness, which I hope I have explained satisfactorily.

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Very interesting! Never knew that the historical past tense 'form' was more polite. – Manjima Jul 27 '12 at 1:35

"Can you X for me?" asks if you have the ability to X under the current circumstances.

"Could you X for me?" asks if you have the ability to X under certain circumstances. If the circumstances are not explicit, the implication is "right now". Using "could" seems more polite than "can".

"Would you X for me?" asks if you are willing to x and assumes you have the ability to do so. This also seems polite.

In the strictest sense, the above three examples are not requests to do something, but questions about ability and willingness to do something.

The most direct, unambiguous request is: "Please X for me?".

For even more politeness, add "Will you" before please. Up the politeness even more by using "would" instead of "will", thus:

Would you please X for me?

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