it used to do sth表示过去常常做什么
它的疑问和否定都有两种表达
疑问：used it to do./ did it used to
否定：it usedn’t to do/ it didn't used to
你的提问就是陈述句it used to be thought that ther earth was flat.采用了第一种疑问式变成的疑问句.以前人们认为地球是平的吗?