Sentence 11:

• Error: The reported speech structure is incorrect. The correct structure is: "Jane asked Brian if he had ever been to South-east Asia, and he said he had."
• Corrected sentence: Jane asked Brian if he had ever been to South-east Asia, and he said he had.

Sentence 12:

• Error: The question form is incorrect. The correct form is: "I don't know why the elevator is not working properly today."
• Corrected sentence: I don't know why the elevator is not working properly today.

Sentence 13:

• Error: The word "bored" should be "boring."
• Corrected sentence: In the end, she quit the job because it was too boring.

Sentence 14:

• Error: The reported speech structure is incorrect. The correct structure is: "He said that he couldn't remember where he had left his car."
• Corrected sentence: He said that he couldn't remember where he had left his car.

Sentence 15:

• Error: The pronoun "they" is unnecessary.
• Corrected sentence: Many couples who both work must hire someone to take care of their children.

Sentence 16:

• Error: The verb "is believing" is incorrect. The correct form is "is believed."
• Corrected sentence: The oil price is believed to be rising again.