_____ you _____(write) a letter when your mother came in ?第一个空是填were,可我不知道为什么.

_____ you _____(write) a letter when your mother came in ?第一个空是填were,可我不知道为什么.
_____ you _____(write) a letter when your mother came in ?
第一个空是填were,可我不知道为什么.

_____ you _____(write) a letter when your mother came in ?第一个空是填were,可我不知道为什么.
came in 是过去时,当母亲进来的时候.
正在进行的动作 也要用 过去时正在进行的动作 -- 即要用 过去进行
人称 是 you, 现在进行是 you are writing, 过去进行是 you were writing,
再把它变成 过去进行时 问句 形式：
Were you writing a letter when your mother came in ?