fall on

1. 向…进攻，袭击，猛烈攻击

例句: They fell on the enemy vigorously.

他们猛烈地向敌人扑去。

2. 落在，(责任、任务、灾难等)落在…身上，由…承担

例句: The responsibility fell upon me.

责任落到了我的肩上。

The bulk of the work will fall on you and me.

大量的工作将落在你和我的头上。

3. 跌倒在…上；掉落在…上

例句: 

The boy fell on hard ground and broke his leg.

这个男孩摔倒在硬地上，把腿给弄断了。

4. 开始，着手

例句: 

The patient fell on trembling.病人开始发抖起来。

fall  in ：陷几（屋顶、天花板）塌陷，坍塌

例句：

It has been three years since the roof started to fall in.从屋顶开始坍塌到现在，3年过去了。

The roof threatened to fall in.屋顶有坍塌的危险。