Use of IF CLAUSE-DHILIP RESOURSE

It is possible to fulfil a condition which is given in the if-clause.

Form

if clause	main clause
Simple Present	will-future  or infinitive  or Modal + infinitive

Examples

If I study,	I will pass the exams.
If you see John tonight,	tell him to e-mail me.
If Ben gets up early,	he can catch the bus.

The if-clause can be at the beginning or at the end of the sentence.

If I study,	I will pass the exams.
I will pass the exams	if I study.

Use

It is theoretically possible to fulfil a condition which is given in the if-clause.

Form

if clause	main clause
Simple Past	would + infinitive  or could + infinitive  or might + infinitive

Examples

If I studied,	I would pass the exams.
If I studied,	I could pass the exams.
If I studied,	I might pass the exams.

The if-clause can be at the beginning or at the end of the sentence.

If I studied,	I would pass the exams.
I would pass the exams	if I studied.

Use

It is impossible to fulfil a condition which is given in the if-clause.

Form

if clause	main clause
Past Perfect	would + have + past participle  or could + have + past participle  or might + have + past participle

Examples

If I had studied,	I would have passed the exams.
If I had studied,	I could have passed the exams.
If I had studied,	I might have passed the exams.

The if-clause can be at the beginning or at the end of the sentence.

If I had studied,	I would have passed the exams.
I would have passed the exams	if I had studied.

Structure

If-clauses can be clause-initial (1), clause-medial (2) as well as clause-final (3).

(1)	If you like, we can catch a movie.
(2)	We, if you like, can catch a movie.
(3)	We can catch a movie if you like.

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Types

Aside from the typical type I, II, III strucure, conditionals can be divided into two categories: 
real and unreal conditionals.

Real Condition

If I have money, I spend it.	Present Real Conditional - type I
If I had money, I spent it.	Past Real Conditional - type I
If I have money, I will/am going to spend it.	Future Real Conditional - type I

Unreal Condition

If I had had money, I would have spent it.		Past unreal Conditional - type III
If I had money, I would spend it.	I think about spending the money TODAY.	Present unreal Conditional - type II
If I had money, I would spend it.	I think about spending the money NEXT WEEK.	Future unreal Conditional - type II

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Special Features

Modal verbs

Main clauses with real conditional tenses can have modal verbs.

If I have money, I can spend it.

You can use could and might instead of would in unreal conditional clauses.

If I had money, I could spend it. (I would be able to spend it.)

If I had money, I might spend it. (I would possibly spend it.)

Mixed Conditionals

Unreal conditionals (type II + III) sometimes can be mixed, that is, the time of the if clause is different from the one of the main clause.

Past --> Present

If I had taken an aspirin, I wouldn't have a headache now.

Past --> Future

If I had known that you are going to come by tomorrow, I would be in then.

Present --> Past

If she had enough money, she could have done this trip to Hawaii.

Present --> Future

If I were you, I would be spending my vacation in Seattle.

Future --> Past

If I weren't flying to Detroit, I would have planned a trip to Vancouver.

Future --> Present

If I were taking this exam next week, I would be high-strung.

The word were in the phrase if I were you is special form. It is known as the subjunctive mood (from the grammatical point of view).
Today you also find the phrase if I was you. Here the Simple Past form of be is used. But there are people who say that this phrase is incorrect and would never use it (mainly Americans). Others say that this phrase can be used.
If I were you I would phone him. - subjunctive mood 
If I was you I would phone him. - Simple Past

will in if-clauses

When the situation or action depicted in the if-clause is a result of the main clause, the will future is used in the if-clause.

He'll pay me $10 if I'll help him do the dishes.  (Doing the dishes is the result of paying ten dollars.)

would in if-clauses

In polite requests would is possible in if-clauses.

It would be nice if you would help me in the kitchen.  (Are you ready to help me in the kitchen?)

Replacing if

If can be replaced by words or expressions with a similar meaning.
The most common are:
as long as
assuming (that)
on condition (that)
on the assumption (that)
provided (that)
supposing (that)
unless
with the condition (that)

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Omitting if

Had I known... (instead of If I had known...)
Were you my daughter,... (instead of: If you were my daughter,...)
Should you need my advice,... (instead of: If you should need my advice,...)

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if vs. when

if and when are interchangeable when the statement of the conditional clause is a fact or a general issue (also known as zero conditonal)

If you heat ice, it melts.

When you heat ice, it melts.

if is used for something that, according to the speaker, might happen.

We can spend the afternoon on the beach if the weather is fine.

when is used for something that, according to the speaker, will happen.

I will clean up the kitchen right away when I'm back from work.

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in case vs. if

in case of can be used to shorten an if-clause as shown below:

If there is a fire, leave the room.

In case of fire, leave the room.

While if expresses a condition (1), in case is used to express a possibility (2).

(1)	I need painkillers if I'm in severe pain.
(2)	I need painkillers in case I'm in severe pain.

The expression just in case is used pretty much the same way.

I got you a pizza just in case you were hungry.  (I don't know whether you are hungry.)