21. fewer， less.fewer指可数名词；less 指价值， 程度或数额。

　　F：The class consists of less than fifteen students.

　　T：The class consists of fewer than fifteen students.

　　F：He spends fewer time on his studies than on his golf game.

　　T：He spends less time on his studies than on his golf game.

　　F：It is true that less girls than boys become engineers.

　　T：It is true that fewer girls than boys become engineers，

　　22.had better， would rather 后跟不带to的不定式。

　　F：You had better to pay attention to the details.

　　T：You had better pay attention to the details.

　　F：I had better not to play tennis this afternoon.

　　T：I had better not play tennis this afternoon.