常见的有以下六种情况：一、假如主句是将来时或表示将来含义时,那么条件和时间状语从句多用一般现在时如： When I grow up, I will be a nurse and look after patients/ I want to be a nurse and look after patients.我长大后要当一名护士，照顾病人。二、假如主句是祈使句，那么从句通常要用一般现在时如：Don't laugh at me when I make a mistake。我犯错误的时候不要笑话我。三、假如主句是含有情态动词的一般现在时，根据需要从句多用一般现在时如：You should be quiet when you are in the reading room在阅览室时应保持宁静。四、 假如 if 的条件状语从句遇到变换间接和直接时 , 祈使句应用 not to.。..如; She said not to close the window常见的时间状语从句的连词有:as soon as,when,while ,as，until 等五、主将从现说的是在含有时间状语从句和条件状语从句的主从复合句中,假如主句的时态是一般将来时，那么从句要用一般现在时。例如：I will tell him about it when he comes.If it is fine tomorrow， we will go outing。六、假如主句的时态是过去将来时，那么从句要用一般过去时 。例如:He said that he would have another try if he had the chance.I knew he could help me if he was free the next day.主要标志有：if 、unless 、until、 when 、as soon as 等If 引导的条件句(if 翻译成”假如”的时候）Ⅰ。（if 从句) 主句v。/ v.三单 will+v。Ⅱ. if+sb+v./ v。三单 can+v.某事很可能发生，条件是可能存在的,主句中某种情况发生的概率也是很高的。如：If you ask him，he will help you．假如你请他帮忙，他会帮你的。If you fail in the exam，you will let him down．假如你考试不及格,你会让他失望的.If you have finished the homework， you can go home．假如你作业做完了就可以回家了。另外，if 从句还表示不可实现的条件或根本不可能存在的条件,也就是一种虚拟的条件或假设。从句多用一般过去时或过去完成时,表示对现在或过去的一种假设.如：If I were you，I would invite him to the party．假如我是你，我会邀请他参加聚会.I would have arrived much earlier if I had not been caught in the traffic．要不是交通堵塞，我本会来...
