Past Unreal Conditional

The Past Unreal Conditional is used to talk about imaginary situations in the past. It is used to imagine different outcomes due to different decisions or actions in different circumstances:
If I had gone out less, I would probably have studied more.
I would have travelled more if I had had more time.
If I had studied French at school, I would have got the job.
Danny would have got the promotion if he hadn't insulted the boss's wife.

With the Past Unreal Conditional only 'if' can be used when the situations discussed are imaginary. 'When' cannot be used.

Conditionals and modals

There are some special conditional forms for modal verbs in English:
would have + can = could have
would have + shall = should have
would have + may = might have
The words "can," "shall" and "may" cannot be used with "would have." Instead, they must be used in these special forms:
If I had gone to England I could have studied interior design.
If he had time, he might have come to meet us.
When Sarah asked me to come to the party, I should have gone.

Unreal Conditional

Sentences in the "unreal" conditional have one part of the sentence that begins with the word "if" and contains a verb in what looks like the past tense (Review past tense) and another part of the sentence which contains the modal verb "would."  The conditional with "would" was taught in Intermediate Lesson 5 (Review conditional). If the part of the sentence with "if" goes first, use a comma.  These sentences are known as "unreal" conditional because they contain conditions that are not likely (probable).  For example, in the sentence below, winning the lottery is not very likely. 

If I won the lottery, I would buy a new car. 
I would buy a new car if I won the lottery.

As I mentioned before, the verb in the part of the sentence beginning with "if" is put in what looks like past tense form.  The only exception is with the verb "to be."  It has traditionally been considered to be grammatically correct to use "were" even with the first person singular ("I") and the third person singular (he/she/it).

If I were rich, I would travel around the world.

Although in the past it was considered bad grammar, now many people use "was" in these cases. You now sometimes even see "was" listed as an alternative in grammar books.

If I was rich, I would travel around the world.

Real conditional

The Real Conditional (The Present Conditional)

Sentences in the present conditional have one part of the sentence which begins with "if" and contains a verb in the present tense (Review present tense) and another part of the sentence in the future with "will" (Review future with "will").  If the part of the sentence with "if" goes first (as in the first example below), use a comma.  The present conditional also known as the "real" conditional because it is generally used with situations that are very likely (probable).  For example, in the sentence below, rain is a definite possibility. 

If it rains, I will use an umbrella. 
I will use an umbrella if it rains.