Question

2．玛丽在七岁时会骑自行车．（could，when ）
Mary could swim when she was five．

Answer 1

分析 Mary could swim when she was five．

解答 答案：Mary could swim when she was five．
表达的是过去的事情用一般过去时，表示能做某事用情态动词can的过去式could，后加动词原形swim，故Mary could swim．when引导的时间状语从句，主语she，故was，故答案是Mary could swim when she was five．

点评 when引导时间从句用过去时，主句用过去进行时：表示在从句动作在过去发生时，主句的动作在当时也正在进行．用于表示在过去有两个动作同时发生的情况，或者一前一后即在过去某动作发生时或正在进行中，又有一个动作发生的情况（反之亦然），通常短暂性动词用过去时，持续性动词用过去进行时．
When I got up，my mum was serving breakfast．
When I was having breakfast，my dad went out of home．