it，them，that，those，one，ones，the one，the ones 的区别：

1. 1). He bought a car and gave it to me.

 2). I bought two pens and gave them to him.

  it/them 代替前面出现过的同一名词, 前单数; 后复数

2. 1). This table is longer than that one.

 2). Do heavy objects always fall faster than light ones?

  one/ones 代替前面出现过的同类名词, 前有定语修饰, 前单数; 后复数

3. 1). The climate of A is as dry as that of B.

 2). The buildings of A is as high as those of B.

  that/those 代替前面出现过的同类名词, 后有定语修饰, 前单数; 后复数

4. 1). ---Which picture is more beautiful?   ---The one on the left. 

 2). These books are more difficult than the ones/those we read just now.

  the one/ the ones 代替前面出现过的同类名词, 后有定语修饰, 前单数; 后复数

 归纳如下：

1.it/them 代替前面出现过的同一名词, 前单数; 后复数

2.one/ones 代替前面出现过的同类名词, 前有定语修饰, 前单数; 后复数

3.that/those 代替前面出现过的同类名词, 后有定语修饰, 前单数; 后复数

4.the one/ the ones 代替前面出现过的同类名词, 后有定语修饰, 前单数; 后复数