The key answer of exercise 4

a)

1. I think if any of us had used it, it would have felt really wrong.
2. If that had happened to me, I might not have jumped.
3. I would have done the same thing.
4. If he hadn't jumped down there, the baby wouldn't be alive.

b)

1. All four sentences are hypothetical because the condition is always in the past so can't be changed.
2. 1-3 refer to the past; 4 refers to the past and the present
3. The conditional (7f) clause is often not used if the condition is understood or obvious. Another example from text 2 in Ex 1B: 'Anybody would have done the same.'

c)

1. past perfect
2. a) modal + have + past participle; b) modal + infinitive