条件句
第一种类型条件句
一、表示普遍真理和客观事实 如：If you heat ice, it melts.
If you pour oil（油） on water, it floats(漂浮）.
从上述的例子可看出，凡表示不受时间的自然法则的条件句，其if-分句和主句的谓语动词都用一般现在时。 （If)simple present + (Main)simple present
二、表示现在习惯的动作 如：If I make a promise, I keep it.
If you go to the movies, what kind of movie do you prefer 在上述表示现在习惯动作的条件句中，if-分句和从句的谓语动词也都是一般现在时:（If)simple present + (Main)simple present
三、表示过去习惯动作
如：If it rained,I went to work by car.
If I made a promise, I kept it.
在上述表示习惯动作的条件句中，if-分句和主句的谓语动词都用一般过去时：（If）+simple past +(Main)simple past.
第二种类型条件句 基本形式：
如：If we catch the 10 o’clock train, we shall(will,can,etc) get there by lunch-time.
If it is fine tomorrow,we can have a picnic somewhere. If you wake up before me, give me a call.
If you meet some friends of mine in London, introduce yourself to them.
由上述诸例可以看出，这一类型条件句中有两个基本形式：一是if-分句用一般现在时表示将来时间，主句用shall/will/can/may等+不定式：
(If)+simple present +(Main)shall/will,etc +其它
另一种基本形式是if-分句用一般现在时表示将来时间，主句用动词祈使句：
(If) simple present+ (Main) imperative
第三种类型条件句
基本形式：If had +ed分词+ Main would(could,might,etc)have +ed分词
如：
If we catch the 10 o’clock train, we would have got there by lunch-time.
We would/could/might have called you if we had known your telephone number.
Exercise:
1)If the temperature (drop) to 0 degrees centigrade, water (freeze).
2)If it (rain), I (go) to work by car. 3)If you had told me to, I (pay) him. 答案：1）drops:freezes 2)rains; go 3)would have paid
