ago vs. before

ago 是現在的過去多久時間before是過去的過去多久時間。

例如：He transferred to another university two years ago.
transferred是過去式，是從說這句話時推算two years ago.
He told me that he had transferred to another university two years before.
told是過去式，had transferred是過去裡的過去(past in the past)，
因為它發生在過去式told之前，此時時態要用過去完成式，並用before,
否則他可能是兩年前或三年前告訴(told)我，加兩年或三年便是四年或
五年了，而非兩年前。

此外，在直接句改為間直接句時，如直接句中有 ago，間直接句中要改為 before.

例如：Direct: Joy said, “I read that novel a year ago.”
Indirect: Joy said that she had read that novel a year before.