曾经 的英语短语怎么写

　　例句：

　　有人曾经认为铅可以变成黄金。

　　It was once thought that lead could be transmuted into gold.

　　人们曾经认为地球是平的。

　　People used to think the earth was flat.

　　我曾经在伦敦居住过。

　　I used to live in London.