“To arrange for an appointment with us, please complete the reply slip and sent it back to us.”

上面這句話很好，很客氣，但是第二分句的動詞sent的時態不對，應該改為send，從過去簡單時態變成現在簡單時態，使兩個等立分句中的動詞時態一致。

在語法上，這叫做時態的一致(sequence of tenses)。下面是幾個指導原則：

Ⅰ 主句(main clause)里的動詞是過去時態的話，名詞從句(noun clause)或副詞從句(adverb clause)里的動詞也要以過去式出現，如：

● Tom said/ that he was busy yesterday.

● Janet wanted to know/ why she was not given a more important job.

● Lily failed/ because she did not submit the project report.

● None could help you/ if you continued being lazy.

● Although David was unwell,/ he went to office as usual.

這原則有兩個例外，即：

(i)當名詞從句表達一種真理或既成事實時，其動詞時態是現在式，如：

● Newton discovered/ that gravitation causes apples to fall.

● Our elders told us/ that experience is the best teacher.

(ii)比較性的副詞從句里的動詞時態有伸縮性，視情況而定，不一定非過去式不可，如：

● Susan valued his advice more/ than she values mine.

● Peter weighed as much/ as I shall weigh after my dinner.

Ⅱ 主句里的動詞時態是現在式時，名詞從句或副詞從句的時態可以是現在式，也可以是其他適合的時態，如：● Jim believes/ that he will get the job.

● The officer has just told us/that his annual leave was approved weeks ago.

● Christine is infuriated/ because she was criticised for her sloppy work.

Ⅲ 形容詞從句(adjective clause)中的時態最自由，最不受拘束，如：● Tom mooted the idea/which some ignored, which others tend to attack, but which many will eventually accept.

這句里的主句動詞是過去式，第一個形容詞從句的動詞雖是過去，第二個里的卻是現在式，第三個里的是將來式。

最后請看看這句話：

“President Clinton stated/ that he will not run in the next presidential election.”

按照常理，名詞從句里的“will not run”要改為“would not run”才能和主句里的過去式“stated”一致。論者把這種現象視為 “特殊情況”，算是給名人的“特權”吧!一般人還是遁常道而行，避免爭議。