1 . The crash ________ (occur) when the crew shut down the wrong engine.(所给词的适当形式填空)

2 . He ________ (sharpen) his knife in order that he cut meat easily . （所给词的适当形式填空）

3 . It suddenly ________ (occur) to him that he had an important meeting to attend the next morning. (所给词的适当形式填空)