Here are suggested answers for the sentences in your exercise:

4. People believe that the island was found by accident.
The island **was discovered accidentally**.

5. The weather was bad, and several cars withdrew from the rally. Due to **the conditions**.

6. He drives so carelessly that he often has accidents.
He's such **a reckless driver**.

7. Jo's training accident meant she couldn’t take part in the race.
Jo's training accident prevented **her from participating**.

8. Let’s have something to drink now.
Shall **we**?

9. Mai met an old friend of hers by chance yesterday.
Mai bumped **into her**.

10. He can’t wait to go on the picnic this weekend.
He is looking **forward to it**.

For the correction section:

1. So hot is the weather that we can’t have gone picnicking as planned.
Correct the phrase: **A**.

2. Are you waiting for the girl that is coming by foot from afar?
Correct the phrase: **C**.

3. What ever did you find in my friend’s room to prove that he had been stealing your watch?
Correct the phrase: **A**.

Feel free to ask if you need further assistance!