Resposta:

* Zero Conditional: If/When....verbo no presente
Algo que sempre eh verdade: If water reaches 100 degrees, the water boils

* First Conditional: If (verbo no presente)....will/could/can/may/might/should (verbo no infinitivo)
Se algo acontecer, a outra coisa sera uma consequencia real: If I get paid, I will go shopping

* Second Conditional: If (verbo past subjunctive)....would/might/could (verbo no infinitivo (simples ou continuous)/ If (verbo no passado).... would/might/could (verbo infinitive simples ou continuous)

Algo hipotetico no futuro (primeira estrutura), ou algo impossivel no presente (segunda estrutura): They might be able to see it if they were more observant. If I met the President, I would be too nervous to speak.

* Third Conditional:  If (past perfect subjunctive)...would/could/might (perfect infinitive)

Situacao acontecida no passado, com um resultado hipotetico se algo mudasse: If I had known how you were going to react, I would have kept my mouth shut.

Explicação: