First Conditional:

   a- Use:

We use the first conditional when we talk about future situations we believe are real or possible.

If it doesn't rain tomorrow, we'll go to the beach.
Arsenal will be top of the league if they win.
When I finish work, I'll call you.

  b- Form:

In first conditional sentences, the structure is usually: if or when + present simple →will + infinitive. 

  Note:

It is also common to use this structure with unless, as long as, as soon as or in case instead of if.

I'll leave as soon as the babysitter arrives.
I won’t want to stay in London unless I get a well-paid job.
I'll give you a key in case I'm not at home.
You can go to the party, as long as you're back by midnight.

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Second Conditional:

  a- Use:

The second conditional is used to imagine present or future situations that are impossible or unlikely in reality.

If we had a garden, we could have a cat.
If I won a lot of money, I'd buy a big house in the country.
I wouldn't worry if I were you.

   b- Form:

The structure is usually: if + past simple → would + infinitive. 

  Note:

When if is followed by the verb be, it is grammatically correct to say if I were, if he were, if she were and if it were. However, it is also common to hear these structures with was, especially in the  he/she form.

If I were you, I wouldn't mention it.
If she was prime minister, she would invest more money in schools.
He would travel more if he was younger.