when I was a child ,i often went to visit my grandpa living in the countryside in the summer.
when she studies ,she often puts a cup of water on the desk.
when I was doing my homework, my friend phoned me.
不要老想着跟什么时态,语言是相通的,所有的选择题其实都是对话的一部分,或者是自我陈述的一部分,你为什么不放在情景里去理解和翻译呢.语言是不能离开情景的.