The key answer of exercise 4

a)

Conditionals

• But for his advice, I would have worked myself into the ground. (paragraph 4)
• If I'd known that statistic when I was learning the ropes, I'd be selling insurance today. (paragraph 6)
• Had I done this, I would have said 'yes' to some great books. (paragraph 9)

Regrets

• I wish I'd spoken to him earlier. (paragraph 4)
• I now regret rejecting some authors who went on to have good careers. (paragraph 9)
• If only I'd known then what I know now. (paragraph 9)

b)

1. If + past perfect + would + have + past participle
2. But for his advice, I would have ... (paragraph 4); Had I done this, I would have ... (paragraph 9)
3. more formal
4. No, they don't. The first clause refers to the past (If l'd known ... (past perfect)), but the second clause refers to the present (... I'd be selling insurance today. (present continuous)). It's called a mixed conditional because it mixes different time periods (past and present).
5. past perfect