verbs - "I need it to lift 2 tons this time." "I don't know if it *could* do." Why do some people use "do" like this?

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

Answer

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)