条件状语从句的时态规则 条件状语从句是由if, when, unless, as long as等引导的从句，用来表示某个条件成立时才能发生的情况。条件状语从句的时态规则如下： 1. 当条件状语从句所表示的动作与主句的动作在时间上同时发生或者是普遍真理时，使用一般现在时态。例如： - If it rains, I will stay at home. - When you heat ice, it melts. 2. 当条件状语从句所表示的动作在主句动作之前发生时，使用过去时态。例如： - If I had known the answer, I would have told you. - Unless you had finished your homework, you couldn't go out to play. 3. 当条件状语从句所表示的动作在主句动作之后发生时，使用将来时态。例如： - If you finish your work early, you will have more free time. - As soon as she arrives, we will start the meeting. 4. 当条件状语从句所表示的动作是客观真理或者是经常性的动作时，使用一般现在时态。例如： - If water boils, it turns into steam. - When it snows, the roads become slippery. 5. 当条件状语从句所表示的动作在主句动作之前的过去的某个时间点发生时，使用过去完成时态。例如： - If you had studied harder, you would have passed the exam. - Unless he had left early, he wouldn't have missed the train. 6. 当条件状语从句所表示的动作在主句动作之后的将来某个时间点发生时，使用将来完成时态。例如： - If you have finished your work by tomorrow, you can take a day off. - As soon as I have completed this project, I will start a new one. 7. 当条件状语从句所表示的动作与主句的动作发生在同一时间点，且主句为祈使句时，使用一般现在时态。例如： - If you see him, tell him I'm looking for him. - When you go to the store, buy some milk. 8. 当条件状语从句所表示的动作与主句的动作发生在同一时间点，且主句为情态动词时，使用情态动词的过去式。例如： 本文来源：https://www.wddqw.com/doc/b975a2765b1b6bd97f192279168884868662b84e.html