Theory:

Conditional sentences are widely used in everyday English, so, this is an important subject.

A conditional sentence has 2 parts: the part which describes the condition, and this can begin with IF or WHEN, and the part that describes the reaction to the condition.
EXAMPLE:
If you pass the exam, (the condition), you will get a certificate. (the reaction)
When you pass the exam, (the condition), you will get a certificate. ( the reaction)

The situation of the condition can either be predictable, in which case WHEN is used, or unpredictable or even hypothetical, in which case IF is used.
EXAMPLE:
When you pass the exam, (you know you are going to pass the exam), you will get a certificate.
If you pass the exam, (you don't know you are going to pass the exam), you will get a certificate. (the reaction)

Conditional sentences cover real and imaginary situations, in the past, the present and the future.
EXAMPLE:
Real
These days, If I have time, I read.(present real because it happens)
When I was younger, If I had time, I read.(past real because it happened)
Next month, If I have time, I will read. (future real because it is possible)

Imaginary
These days, If I had time, I would read. (present imaginary because I don't have time)
When I was younger, If I had had time, I would have read.(past imaginary because I didn't have time)
Next month, If I had time, I would read. (future imaginary because I will not have time)

There are 4 types of conditional sentences: zero conditional, first conditional, second conditional and third conditional.

In a zero conditional sentence, the result is always true.
EXAMPLE:
If you heat ice, it melts. (always)
(The if part (heat ice) is in the present and the reaction part (melts) is also in the present)

A first conditional sentence talks about the future and describes a situation which could happen.
EXAMPLE:
If it rains tomorrow, (which is possible), I will stay at home.
(The if part (rains) is in the present and the reaction part (will stay) is in the future)

A second conditional sentence talks about an imaginary situation in the present.
EXAMPLE:
I don't have a ticket, but, If I won the lottery, (which is impossible), I would buy a car.
(The if part (won the lottery) is in the past and the reaction part (would buy) is in the future)

A third conditional sentence talks about an imaginary situation in the past.
EXAMPLE:
I didn't buy a ticket, but, If I had won the lottery, (which was impossible), I would have bought a car.
(The if part (had won the lottery) is in the past perfect form and the reaction part (would have bought) is in the present perfect form)