14. If you had driven more carefully, we wouldn't have had that accident.

• Had is already the correct answer. This sentence is in the second conditional, which talks about hypothetical situations in the past and their consequences in the past.

15. If he were late, he would apologise.

• Were is already the correct answer. This sentence is in the third conditional, which talks about hypothetical situations in the present or future and their impossible consequences.

16. If the weather is fine, we will go picnic.

• Should is a good alternative here.
  • The original sentence uses the zero conditional, which talks about general truths or habits. However, "if" can also be used to make suggestions in a more polite way.
  • "Should the weather be fine" suggests a possibility and a potential action based on that possibility.

Were he to be late ,he would apologise.

Should the weather is fine, we will go picnic .