in 我不说了~可以作为介词词组来用.
ago 就是多少时间之前
比如 我来加拿大半年了
I've been living in Canada since half a year ago.
after:在……之后 通常加时间 或一件事情.
比如,我完成作业后才回家：
I went back home after I finished my homework.
(当然这一句用 not.until 句更好)
before:在……之前
比如,同一句例句我可以表达成：
Before I went back home I finished my homework.
that's it!