1. When Dave left school, he worked for several years in a zoo.
唯一准则答案是：For several years after Dave left school, he worked in a zoo.
我的句子：After Dave left school, he worked for several years in a zoo.
请教老师，我的答案是不是正确？第二，解题人把for several years放置在after之前，意图何在？
4. He did all his work. He went to bed.
He had done [did?] all his work by the time he went to bed.
请教老师除去用had done，能否用did？谢谢老师解说。
这项训练应该是某个教程的课后训练，需要学生学会after和by the time引导时间状语从句的使用方法。
第一句的题句本身有问题，时间状语应该放在地址状语之后，故第一句的题句应改为：
When Dave left school, he worked in a zoo for several years.
根据训练的需要，最简单的正确答案就是直接用after替换when:
After Dave left school, he worked in a zoo forseveral years.
时间状语从句可以放在句末，也可以放在句首。放在句首一方面有强调状语有哪些用途，其次可以承上启下，譬如前面的内容是讲述主语在学校的状况时，状语放在句首则非常自然地过渡到离校后的状况。假如前文不是谈论主语在学校的状况，则时间状语最好放在句末：
Dave worked in a zoo for several years after he left school.
这种训练没有所谓唯一正确答案，教程只给了一个参考答案而已。
其实，教程给出的参考答案是不妥的。逻辑剖析可知，after从句是修饰主句的句子状语，而for介词短语是修饰主句谓语动词的谓体状语。谓语状语可以前置表示强调，但不可以用逗号隔开。而句子状语可以坐落于句末或句首，坐落于句首时，一般用逗号。所以不应该将谓体状语和句子状语同时前置。
假如将for介词短语理解为句子状语，那样after从句则非常可能被理解为是修饰several years的，即这几年是毕业之后的，但未必是紧接毕业时间，可能是毕业后失业了二年，然后才工作了三年。所以会产生歧义。
总之，教程的答案并非最好答案。
He did all his work. He went to bed.
这项训练是需要学生用by the time连接句子。那样你只有一种选择：
By the time he went to bed, he had done all his work.
He had done all the work by the time he went to bed.
by时间状语表示主句谓语的动作在by表示的这个时间之前已经发生。说话人关心的时间是by所表示的时间，所以假如主句谓语是动态动词表示事件意义，则应该用完成体，而不可以用一般体。这个句子用did为语法错误。