1st Conditional: if + present simple=will + infinitive We use the first conditional to talk about a possible event in the future. If you invite him, he'll come.
2nd Conditional: if + past simple=would + infinitive We use the second conditional to talk about an impossible event in the present/future or to give advice. If we all recycled more, we would help our planet.(but we don't) If I were you, I would eat less sugar.(advice)
3rd Conditional: if + past perfect=would have + past participle
We use the third conditional to talk about imaginary event in the past. If I had studied more, I would have passed the exam.(but I didn't so I failed)