3)across, through

across和through均可表示“从这一边到另一边”，但用法不同。Across的含义与on有关，表示动作在某一物体的表面进行。Throgh的含义与in有关，表示动作是在三维空间进行。例如：

The dog ran across the grass.

The boy swam across the river.

They walked through the forest.

I pushed through the crowds.

4) in front of, in the front of

in front of 表示“在某人或某物的前面”，在某个范围以外;in the front of 表示“在……的前部”，在某个范围以内。例如：

There are some tall trees in front of the building.

The teacher is sitting in the front of the classroom.

3. 介词的固定搭配

介词往往同其他词类形成了固定搭配关系。记住这种固定搭配关系，才能正确使用介词。

(1)介词与动词的搭配

listen to , laugh at, get to, look for wait for, hear from, turn on, turn off, worry about, think of, look after, spend…on…, 等。

(2)介词与名词的搭配

on time, in time, by bus, on foot, with pleasure, on one’s way to, in trouble, at breakfast, at the end of, in the end等。

(3)介词与形容词的搭配

be late for, be afraid of, be good at, be interested in, be angry with, be full of, be sorry for等。

4. 连词的功能

用来连接词，短语，从句或句子的词叫连词。连词是一种虚词，在句中不能单独使用。连词可分为两类：并列连词和从属连词。

5. 并列连词

并列连词用来连接具有并列关系的词，短语或句子。常见的并列连词有：

(1)表并列关系的and, both…and, not only…but also, neither…nor等。

(2)表选择关系的or, either…or等。

(3)表转折关系的but, while等。

(4)表因果关系的for, so等。

6. 从属连词

从属连词用来引导从句。常见的从属连词有：

(1)引导时间状语从句的after, before, when, while, as, until, till, since, as soon as等。(2)引导条件状语从句的if, unless等。

(3)引导原因状语从句的because, as, since等。

(4)引导目的状语从句的so that, in order that等。

(5)引导让步状语从句的though, although, even if等。

(6)引导结果状语从句的so that, so…that, such…that等。

(7)引导比较状语从句的than, as…as等。

(8)引导名词从句的that, if , whether等。

7. 常用连词的用法辨析

(1) while, when, as

这三个连词都可引导时间状语从句，但用法有所不同。

1) 当某事正在进行的时候，又发生了另一件事。While, when, as 都可用来引导表示“背景”的时间状语从句。例如：

As/When/While I was walking down the street I noticed a police car.

2) 当两个长动作同时进行的时候，最常用的是while。例如：

While mother was cooking lunch, I was doing my homework.

3) 当两个动作都表示发展变化的情况时，最常用的是as。例如：

As children get older, they become more and more interested in things around them.

4) 当两个短动作同时发生时，或表示“一边…一边…”时，最常用as。例如：

Just as he caught the fly, he gave a loud cry.

She looked behind from time to time as she went

5) 当从句的动作先于主句的动作时，通常用when。例如：

When he finished his work, he took a short rest.

6) 当从句是瞬间动作，主句是延续性动作时，通常用when。例如：

When John arrived I was cooking lunch.

(2)as, because, since , for

这四个词都可表原因，但用法有区别。