1. the + 序数词，表示第几；
a+ 序数词表示又一，再一
例如：The cake is delicious, and I would like a second one. 蛋糕很好吃，我想再吃一块。
2. a number of ... 大量的...;
the number of... ......的数量；
例如：A number of students like playing computer games.
The number of students is about 1,500 in our school.