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

上面这句话很好，很客气，但是第二分句的动词sent的时态不对，应该改为send，从过去简单时态变成现在简单时态，使两个等立分句中的动词时态一致。

在语法上，这叫做时态的一致(sequence of tenses)。下面是几个指导原则：

Ⅰ 主句(main clause)里的动词是过去时态的话，名词从句(noun clause)或副词从句(adverb clause)里的动词也要以过去式出现，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.

这原则有两个例外，即：

(i)当名词从句表达一种真理或既成事实时，其动词时态是现在式，如：

● Newton discovered/ that gravitation causes apples to fall.

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

(ii)比较性的副词从句里的动词时态有伸缩性，视情况而定，不一定非过去式不可，如：

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

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

Ⅱ 主句里的动词时态是现在式时，名词从句或副词从句的时态可以是现在式，也可以是其他适合的时态，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 形容词从句(adjective clause)中的时态最自由，最不受拘束，如：● Tom mooted the idea/which some ignored, which others tend to attack, but which many will eventually accept.

这句里的主句动词是过去式，第一个形容词从句的动词虽是过去，第二个里的却是现在式，第三个里的是将来式。

最后请看看这句话：

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

按照常理，名词从句里的“will not run”要改为“would not run”才能和主句里的过去式“stated”一致。论者把这种现象视为 “特殊情况”，算是给名人的“特权”吧!一般人还是遁常道而行，避免争议。