Ответ:

1. Nick wouldn't have broken his leg unless he had climbed ladder

2. Mr. Davidson could have avoided accident if he had driven carefully

3. John Smithson  could have won the race if he had run faster

4. Chris wouldn't have cut his finger unless he had played with knife

5. Mary wouldn't have had a terrible headache if she had taken aspirin

6. Cat wouldn't have eaten fish unless Mrs. Claydon had  left it on the table

Объяснение:

unless- это эквивалент if not

все предложения 3го условия, т к относятся к прошлому, которое уже нельзя изменить