when I was four ,I often played with my sister .(当我四岁的时候,我经常和我的姐姐玩耍)后面用的是过去式
when he was back ,he was listening to music .(当他回来时,他正在听歌儿)后面用的是过去进行时
when he was six ,he had joined us .(当他六岁时,就已经加入我们了)后面用的是过去完成时
至于动词原型,要看句子的成段,如果下一句已经是下一个主题了,便可以用原型
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