Sentences containing conditional clauses like if, unless, provided that are called conditional sentences. They express four types of conditions and each type has different structures and meanings.
|Zero||always real and possible||simple present, simple present||If it rains, we need an umbrella.|
|First||possible situations in the present or future.|| |
simple present, will/shall/can/may+v1
simple present, imperatives
If she meets me, I will help her.
If you want to achieve success, work hard.
|Second||unlikely but possible situation in the present or future||simple past, would/could/might+V1||If i were a bird,I could fly high.|
|Third||unreal situations in the past||past perfect, would/could/might+ have+V3||If they had invited me, I could have come.|
Had+ subject+ past participle: