Page 272 - Practical English Usage 3ed - Michael Swan, Oxford

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if (7): other structures found in spoken English would in both clauses
Conditional would is sometimes used in both clauses of an if-sentence. This is very informal, and is not usually written. It is common in spoken American English.
It would be good if we'd get some rain.
How would we feel if this would happen to our family?
For if... would in polite requests, see 260.4. 'd have. .. 'd have
In informal spoken English, if-clauses referring to the past are sometimes constructed with 'd have. This is frequently considered incorrect, but happens quite often in educated people's speech. It is not normally written.
I f I'd have known, I'd have told you.
It would have been funny ifshe'd have recognised him.
had've and would've
Instead of the contracted'd in these structures, full forms are sometimes used for emphasis or in negatives. Both had and would occur. The following are genuine examples taken from conversation.
I didn't know. But ifI had've known ...
We would never have met ifhe hadn't have crashed into my car.
I f I would've had a gun, somebody might have got hurt.
Ifyou wouldn't have phoned her we'd never havefound out what was
happening.
mixed tenses
Sometimes a simple past tense is used with ifwhere a past perfect would be normal. This is more common in American English.
If I knew you were coming I'd have baked a cake.
If I had the money with me I would have bought you one.
IfI didn't have my walking boots on I think I would have really hurt myfoot.
if (8): other words with the same meaning
Many words and expressions can be used with a similar meaning to if, and often with similar structures. Some of the commonest are imagine (that), suppose (that), supposing (that) (used to talk about what might happen), and providing (that), provided (that), as/so long as, on condition (that) (used to make conditions).
Imagine we could all fly. Wouldn't that be fun!
Supposing you'd missed the train. What would you have done?
You can borrow my bike providing/provided you bring it back.
I'll give you the day offon condition that you work on Saturday morning. You're welcome to stay with us as/so long as you share the expenses.
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if (7): other structures found in spoken English 262
For suggestions with SIIPPOse, slIpposillg and what if, see 571.
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