4

{| class="wikitable"
|-
! if-clasuse!! main-clause
|-
| If you hadn’t been so rude || Mrs Tripp wouldn’t have given you this extra work.
|-
| If Mike Lowe hadn’t told that stupid old story || I wouldn’t have interrupted him.
|-
| If you had just kept quiet || you’d have been free all weekend.
|-
|}

If-clause: past perfect = had/had not + third form

Main clause: wouldn’t + have + past participial/third form

The first sentence is not very likely but possible. 

The second sentence describes an impossible event.

If the Natives hadn’t been so friendly, they would have killed Columbus and his crew. 

If Columbus had sailed to North America, he would have arrived in New England.

If Columbus had arrived in New England, it would have become a Spanish colony.

If Europeans hadn’t come to America, millions of Native Americans wouldn’t have died of European diseases.

If we hadn’t known corn and potatoes, Europeans wouldn’t have found them in America.

If the Wampanoag hadn’t shown the Pilgrims how to grow corn, all would have died in their first winter.

If Patuxet hadn’t been empty, the Pilgrims would have had to build their village somewhere else.