▲主从复合句中时态的一致主要有以下几种情况：
【主将从现】
在以when，after，as soon as等引导的时间状语从句以及以
if，unless等引导的条件状语从句中，如果主句是一般将来时，从句则用一般现在时。
I'll tell her the good news when she comes back.
当她回来的时候，
我将把这个好消息告诉她。
If it doesn't rain, he will come here on time.
如果不下雨，他会按时来这儿的。
※如果主句是祈使句，那么从句通常要用一般现在时。
如：
Don’t laugh at me when I make a mistake.
我犯错误的时候不要笑话我。
※如果主句是含有情态动词的一般现在时，根据需要从句多用一般现在时。
如：
Y ou should be quiet when you are in the reading room.
在阅览室时应保持安静。
【主过从过】
在含有宾语从句的主从复合句中，当主句的谓语动词为过去时态时，从句须用表示过去的某种时态。例如：
He asked when they would go to the party.
他问他们什么时候将去参加聚会。
Miss Green said she had been to Beijing before.
格林小姐说她以前曾经去过北京。
【主现从任】
在宾语从句中，当主句的时态为一般现在时、现在进行时、现在完成时等时态时，后面从句时态不受主句限制，可根据情况，选用各
种适合的时态。例如：
Do you know when we'll have a football match?
你知道我们什么时候举行足球赛吗？
Y ou know he has gone to Shanghai.
你知道他已去上海了。
▲“时态后移”(tense back—shift)”，指的是复合句中的主句动词为过去时或过去完成时时，其从句动词如为陈述语气，则时态应作相应的后移，以便与主句动词在时间上取得逻辑上的呼应，即从句动词现在时应后移为过去时；过去时应后移为过去完成时。
The staff members were asked to arrive a few minutes earlier before the meeting started.
(这里before从句可用完成时态吗？)
▲before从句的过去完成时都是属于虚拟语气，表示从句事件都没有实现。
The bell rang before we had finished our work．
I met George before he had heard the news．
Tom called to see me before l had had breakfast．The thief was caught before he had disposed of t11e sto1en goods．
在语义上，这四句分别等价于：
When the bell rang，we had not finished our work．When l met George，he had not heard the news．
When Tom called to see me ，I had not had breakfast．When the thief was caught，he had not disposed of the
stolen goods．
