例句：

　　有人曾經(jīng)認(rèn)為鉛可以變成黃金。

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

　　人們曾經(jīng)認(rèn)為地球是平的。

　　People used to think the earth was flat.

　　我曾經(jīng)在倫敦居住過。

　　I used to live in London.