自学英语,When did it start raining.一个句子里的start doing是怎么回事?

自学英语,When did it start raining.一个句子里的start doing是怎么回事? It's raining .when did it start? -It's raining!When did it start? When did it start? when did you start doing it?请翻译 when did it start? 解释 要解释说明.（32）--It's still raining!When did it start?---I don't When did it start?About ______ ago. 是when did it started还是when did it start? When did it start?为什么did用过去式? 英语start to do 和 start doing的区别? when did you start to______?