11. She speaks English than her sister. (fluent) 12. The cat walked across the keyboard. (careful) 13. He arrived than expected. (early) 14. This is the book I've ever read. (interesting) 15. They sang than the other choir. (bad)

Here are the completed sentences:

11. She speaks English more fluently than her sister. (fluent)
12. The cat walked carefully across the keyboard. (careful)
13. He arrived earlier than expected. (early)
14. This is the most interesting book I've ever read. (interesting)
15. They sang worse than the other choir. (bad)