名词性从句中主句和从句的时态问题

英语自学网

（1）宾语从句中主句和从句的时态保持一致，但如果从句中表示提示的是客观现象，虽然主句是过去时态，从句仍用一般现在时。如：
        　　Hello,I didn’t know you were in London. How long have you been here?
        　　The teacher told us that light travels at a very high speed.
        （2）主语从句作主语相当于单数第三人称作主语，谓语动词用单数，如果由and 连接两个或两个以上的主语从句作主语时，谓语动词用复数；由两个或多个连接词引导一个主语从句，谓语动词用单数。如：
        　　When the meeting will begin has not been decided yet.
        　　When they will start and where they go have not been decided yet.
        　　When and where the meeting will begin has not been decided.