simon thought his computer was broken until his little broth

simon thought是主句里面的主语和谓语,his computer was broken是宾语从句, until his little brother pointed out .是时间状语从句,这个时间状语从句又带了一个宾语从句.
until引导时间状语从句时,主句不用过去完成时.因为until与它所带的成分只表示一个过去的时间点.如：
He didn't get up until 8:00.（这个until是介词,until 8:00是介宾短语,表示一个过去的时间点.）
He didn't answer the question until the teacher told him to do so. （这里的until是个连词,引导一个时间状语从句.从句中虽然有一个动作,但它的意义不在动作的本身,而是借助这个动作表示一个过去的时间点.）
祝你学习进步,更上一层楼! (*^__^*)
不明白的再问哟!请记得采纳,多谢!