'Would' and 'would have' (I would do it if I could...)

[Prasunkumar]

Someone who knows better please tell me the difference between simple 'would' and 'would have'. When to use which guy? When to 'I would do it if I could' and when 'I would have done it if I could have'? Please answer responsibly. Your post will be registered with honour and it can influence my future speech and writing. Thanks.

Prasun.

[Mister Micawber]

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'I would do it if I could'-- a present or future unlikely possibility.
'I would have done it if I could have' -- a past impossibility.
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Native English teacher at Mister Micawber's

[Prasunkumar]

Hi Micawber,

How's good ol' Devid? Haha. Nice name. But sir, is this problem such a cinch? With utmost regards, I don't think so. In many cases 'would' is used while not denoting 'a present or future unlikely possibility' and 'would have' is used not in case of 'a past impossibility'. What do you think? Take your sweet time. I'm not rushed. I've been living with this problem for quite some time; some time more will hardly make any difference.
Take care,
Prasun.

[Mister Micawber]

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Yes, it is a cinch, Prasun. It is not a problem for me at all. Please present examples to support your contention.
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Native English teacher at Mister Micawber's

[Prasunkumar]

Sir observe the following sentence:

I decided not to help Devid. I would if my help could do him any good but I was sure it could not.

Can 'would have' and 'could have' replace 'would' and 'could' respectively in this sentence? If not, why? If yes, again, why. Please tell me. I'm trying to get rid of this problem once for all. I'm feeling envious of you that you don't find it a problem at all! Honest!

Regards,
Prasun.

[Mister Micawber]

.
This sentence speaks of a hypothetical present and future. If you use 'would have' /'could have', it would speak of a hypothetical past.
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Native English teacher at Mister Micawber's

[Prasunkumar]

Thanks for your post Mr. Micawber. It helped. Take care. Will talk to you later.

Prasun.