Conditional sentences

There are three types of the if-clauses.

type	condition
I	condition possible to fulfill
II	condition in theory possible to fulfill
III	condition not possible to fulfill (too late)

Form

type	if clause	main clause
I	Simple Present	will-future (or Modal + infinitive)
II	Simple Past	would + infinitive *
III	Past Perfect	would + have + past participle *

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.

Type I
Use

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.

Type II
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.

Type III

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.

Special Types and Structures

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 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

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)

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 conditonal)

If you heat ice, it melts.

When you heat ice, it melts.

http://www.englisch-hilfen.de/en/complex_tests/if_clauses1/index.php

http://www.englisch-hilfen.de/en/exercises_list/if.htm