Examples (if-clause at the beginning)

type	if clause	main clause
I	If I study,	I will pass the exam.
II	If I studied,	I would pass the exam.
III	If I had studied,	I would have passed the exam.

Examples (if-clause at the end)

type	main clause	if-clause
I	I will pass the exam	if I study.
II	I would pass the exam	if I studied.
III	I would have passed the exam	if I had studied.

Examples (affirmative and negative sentences)

type		Examples
		long forms	short/contracted forms
I	+	If I study, I will pass the exam.	If I study, I'll pass the exam.
-	If I study, I will not fail the exam. If I do not study, I will fail the exam.	If I study, I won't fail the exam. If I don't study, I'll fail the exam.
II	+	If I studied, I would pass the exam.	If I studied, I'd pass the exam.
-	If I studied, I would not fail the exam. If I did not study, I would fail the exam.	If I studied, I wouldn't fail the exam. If I didn't study, I'd fail the exam.
III	+	If I had studied, I would have passed the exam.	If I'd studied, I'd have passed the exam.
-	If I had studied, I would not have failed the exam. If I had not studied, I would have failed the exam.	If I'd studied, I wouldn't have failed the exam. If I hadn't studied, I'd have failed the exam.

* We can substitute could or might for would (should, may or must are sometimes possible, too).

I would pass the exam.

I could pass the exam.

I might pass the exam.

I may pass the exam.

I should pass the exam.

I must pass the exam.

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.

Types:

Aside from the typical type I, II, III structure, 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

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.)

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)

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,...)

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 conditional)

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.

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