Conditional ("if") sentences normally have two parts.
One part shows a result and the other shows a condition
on which the result depends. The condition is normally
preceded by if. In "He'd be angry if he hadn't gotten
what he wanted," the result is "he'd be angry" and the
condition (introduced by "if") is "he hadn't gotten
what he wanted."

There are two main types of conditional sentences: real
and unreal. Real conditional sentences refer to situations
that are either true or possible. Unreal conditionals refer
to situations that are untrue, impossible or hypothetical;
conditional sentences of this type are often described as
being contrary to fact.

Let's look at another possibility for unreal conditionals.

There are several types of unreal conditional sentences.
They differ according to the time that they refer to, but
they are the same in one way: the situations that they
show are unreal, hypothetical, and contrary to fact.

Another type of unreal conditional is used for mixed
times (that is, different times for the condition and the
result). There are several types of mixed-time unreal
conditionals. One has the condition in the past and
the result in the present or future:

 

Examples:

If she had left yesterday, she would be
at home today. / She would be at home
today if she had left yesterday.

(She didn't leave yesterday, so she isn't
at home today.)

 

If he had learned English when he was
very young, his pronunciation would be
perfect. / His pronunciation would be perfect
if he had learned English when he was
very young.

(His pronunciation isn't perfect because he
wasn't very young when he learned English.)

 

If he had starting saving his money earlier,
he would be able to buy a car sometime
this year. / He would be able to buy a car
sometime this year if he had started saving
his money earlier.

(He won't be able to buy a car at any time
this year because he didn't start saving his
money early enough.)

 

If I had been born in 1940, I would be
61 years old on my next birthday. /
I would be 61 years old on my next birthday
if I had been born in 1940.

(I won't be 61 years old on my next birthday
because I wasn't born in 1940.)

 

Special Note:

A past condition can have a present or future result,
but (logically) a present or future condition can't have
a past result.