有人最近問了我這個(gè)問題：

其實(shí)里面有這樣一個(gè)重要的句型：it has been+時(shí)間段+since+名詞+ 動(dòng)詞

這個(gè)結(jié)構(gòu)中，如果最后的動(dòng)詞是瞬間動(dòng)詞，表肯定意義，比如：It's been five years since I moved here（我搬到這兒已經(jīng)5年了）。
如果動(dòng)詞為延續(xù)性動(dòng)詞，那就表否定意義，比如：It's been five years since Michael was a teacher.（麥克不當(dāng)老師已有5年）。

那如果表示“麥克當(dāng)老師五年了”這樣的肯定意義，怎么說(shuō)呢？

把 was 改成瞬間動(dòng)詞即可——It's been five years since Michael became a teacher.

再回到一開始的句子：It's been five years since I smoked.
這里 smoked 是延續(xù)性動(dòng)詞（抽煙的這種狀態(tài)），句意表否定，表示"我不抽煙已經(jīng)5年了"。

那怎么表示“我抽煙抽了5年”呢？

把 smoked 的位置改成瞬間動(dòng)詞即可：It's been five years since I began to smoke.