Across可指从一个地方到另一个地方的移动方向。

P：表移动的动词 + across + n
The girl ran across the yard.  
那个女孩穿跑过了院子。

Across之前的动词通常是：crawl, drive, go, limp, move, ride, run, swim, walk

Across可指在某个地方的对面

P：v + across + n
My friend lives across the street.  
我的朋友住在街的对面。

Across from的意思是“对着……”

P1： v + across from + n
My assistant's office is across from mine.
我助理的办公室和我的正对着。

My secretary sits across from me.  
我的秘书面对着我坐下。

P2：v + across + n +from + n
My assistant's office is across the hall from mine.  
我助手的办公室和我的是对门。

Across 和 all across的意思是“在每一个地方”、“在所有的地方”。

People across the world are using the Internet.
世界上的所有人都在用因特网。

There is a heat wave all across the country.
酷暑席卷全国。

词组：

across the board：全面地
Everyone got a raise in salary: there was a wage increase of three percent across the board.
每个人都加了工资，即每个人每周增加百分之三的薪水。

come across（不可分）：偶遇、意外碰到、无意中发现
I came across this old picture of you when I was looking for some documents.
当我找文件的时候，我无意间找到了你的这张老照片

come across（不及物词组）：被听众被理解
The banquet speaker was not sure how well he came across.  
宴会的主持人不知道观众理解了自己多少。

run across（不可分）： 不经意间找到某物
I ran across a letter you wrote to me when we were children.  
我无意间找到了一封你写的信，那时我们都不大。

get (sth) across to：让……被理解
The young girl tried to get it across to her boyfriend that she was not ready to get married.
这个小女孩试图让自己的男友知道自己并不准备结婚。