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Sometimes, instead of saying "could/can" or "would/will" (the two most common I've heard), some people say "could/can do" or "would/will do".

Instead of:

I don't know if it can.

I sometimes (but uncommonly) hear:

I don't know if it can do.

Say them with a mild emphasis on the italicized can and you'll be saying it as I hear it.

As this is a little hard to express to someone who has never come across it before, I hope someone who sees this knows what I mean. Why is that extra "do" sometimes used?

Additional note: Here are three examples that in jwpat7's opinion may be similar to or represent what Daniel δ refers to:

I have never crossed the Himalayas, though I might have done. - Christopher Hughes, 2006

...when I could get a vote for Mr. Charlesworth, I did do. ... I made no regular canvass, [but] where I could get a vote, I did do. - Slade testimony, 13487-8, 1860

"... When needed I will turn myself in too. And if I can do, you can do too!" - Oblivion in Progress, 2011, pp. 67-68

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2  
"Can do, can do, this guy says the horse can do." –  Peter Shor Jul 22 '11 at 11:45
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This usage is regional. –  GEdgar Jul 22 '11 at 15:57
    
The song I quoted three comments above, from Guys and Dolls, was written by Frank Loesser, a New Yorker. I'm fairly sure it's also regional in the U.S. I know I've heard it here. –  Peter Shor Jan 8 '12 at 19:25
    
Can you give us some more detail to go on? Who says it? In what context? –  Pitarou Jan 9 '12 at 9:59
3  
If I get married in America, I'll say "I". –  Kris Jan 12 '12 at 7:18
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2 Answers 2

up vote 14 down vote accepted
+100

As far as I am aware, this is a primarily UK usage. The difference comes in how American and British english speakers handle verb phrase ellipsis, a construction in which a verb phrase is left out because it's implied or repeated. Consider this example (taken from this blog post, which contains an excellent discussion of the issue):

A: I ate all the chocolate, even though I shouldn't have [eaten all the chocolate].

In such situations, American speakers tend to simply delete the elided phrase:

I ate all the chocolate, even though I shouldn't have.

But British speakers tend to replace the elided phrase with the pro-verb do: (A pro-verb is the verbal counterpart of a pronoun, also called a propredicate. They are both part of the general category of pro-forms, or words that take the place of a specific part of speech.)

I ate all the chocolate, even though I shouldn't have done. (BrE)

Note that American English also has a pro-verb construction, but they use do so instead of do:

I ate all the chocolate, even though I shouldn't have done so.

According to this source, all three forms are acceptable in British English, while only do so and complete deletion are okay in standard American English. (There are a few American dialects that do use pro-verb do - see this article for a discussion - but the vast majority do not.)

The phenomenon occurs very freely - with almost any elided verb-phrase that's introduced by an auxiliary. Thus, in British English you'll hear I might do, I can do, I will do, I could do, I would do, etc, while the more likely American equivalents would be I might, I can, I will, I could, I would.

Returning to your specific example, we can see that it can be expressed in three possible ways, which vary in terms of usage.

I need it to lift 2 tons this time...

(1) I don't know if it can. (common in AmE and BrE)

(2) I don't know if it can do. (impossible in AmE, common in BrE)

(3) I don't know if it can do so. (possible in both AmE and BrE)

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Because "do" is a regular pro-verb in English.

"Do you think this is right?" "Yes I do".

"Do you want some melon?" "Yes I do".

When there is an auxiliary such as "will" or "can" it is optional, but can still occur.

This probably developed from "do" as an emphatic auxiliary ("I do want some"), through its use as a negative and interrogative auxiliary, which eventually became compulsory for most verbs ("Do you know?" replacing earlier "Know you?")

Edit: I realise, rereading the question, that you are asking about the doubt implied by the usage. It usually (though not always) has a certain doubt about it, but the meaning is not

I don't know if it can

but rather

It can, but there might be a better way to do it.

So it is still asserting the statement, but there is an implied "but" or "if" following.

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I don't know which aspect (pro-verb vs. doubt) interests Dδ, but the tail of the question title and examples I added address the former. –  jwpat7 Jan 13 '12 at 18:07
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