条件句第一种类型条件句1、 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，凡表示不受时间限制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 present2、 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 present3、 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 oclock 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 oclock 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