Second conditional. The second conditional is used to imagine present or future situations that are impossible or unlikely in reality. The structure is usually: if + past simple >> + would + infinitive. Ex.: 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.
When if is followed by the verb be, it is grammatically correct to say: if I were, if he were, if she were if it were However, it is also common to hear these structures with WAS, especially in the he/she form. Ex.: 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.
First conditional. We use the first conditional when we talk about future situations we believe are real or possible. In first conditional sentences, the structure is usually: if/when + present simple >> will + infinitive. Ex.: 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.
Instead of if, it is also common to use this structure with: unless, as long as, as soon as, in case. Ex.: I'll leave as soon as the babysitter arrives. I don'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.
Zero conditional. We use the zero conditional to talk about things that are generally true, especially for laws and rules. The structure is: if/when + present simple >> present simple. Ex.: If I drink too much coffee, I can't sleep at night. Ice melts if you heat it. When The sun goes down, it gets dark.