The first is the zero conditional

It expresses conditions that are always true, with automatic or habitual results.  

if-clause main clause
form if + present simple present simple
example If you heat ice, it melts.

Doğa Yasalarının veya Genel Kuralların İfadesinde Kullanılır.

The second is the first conditional:

It expresses a condition which is possible and has a probable result.

if-clause main clause
form if + present simple will + base form of verb
example If you leave early, you will catch the train.

Olanaklı ancak koşula bağlı gerçek durumların ifadesinde kullanılır

The third is the second conditional

It express a hypothetical condition and its possible result, or an unreal situation.

if-clause main clause
form if + past simple would + base form of verb
example a) If I had enough money, I would eat in a restaurant.
b) If I was younger, I would change my life.

Varsayıma bağlı bir koşulun ve ona ait olanaklı bir sonucun ifadesinde kullanılır

The fourth is the third conditional:
It expresses imaginary situations about the past.

if-clause main clause


form if + past perfect would have + 3rd form
(had + 3rd form)
example If I had seen you, I would have said hello.

Ne koşulun ne de buna bağlı sonucun gerçekleşmesi söz konusudur. Tümüyle varsayımsaldır.

If I Were A Cat

by Mary Byrd

If I were a cat, I'd be the best cat
That I possibly could be.
I'd come as a present
Wrapped with a big bow.
I'd delight a little girl
And possibly a beau.
I'd be cute and furry
Cuddly and fat.
I'd rub on your leg
And sit in your lap.
Lick your sweet face
And tickle your feet.
My favorite foods would be;
A bowl full of milk,
And a stinky old rat. I'd smack my lips
And stiffle a burp.
But best of all
I'd bring joy to all
When I played with my toys
And purred in your ear.