31. a number of, the number of

a number of 许多，谓语动词用复数；
the number of… ……的数目，谓语动词用单数。例：The number of students is increasing.

32. in front of, in the front of

in front of 范围外的前面；
in the front of 范围内的前面 例：In the front of the room sits a boy.

33. of the day, of a day

of the day 每一天的，当时的，当代的，例：a famous scientist of the day；
of a day 暂时的，不长久的。

34. three of us, the three of us

three of us 我们（不止三个）中的三个；
the three of us 我们三个（就三个人）例：The three of us - Tom, Jack and I went to the cinema.

35. by bus, on the bus

by bus 表手段，方式，不用冠词，例：They went there by bus.
on the bus 表范围。