Question

i don't know who will come

这是个宾语从句对吧，从句的时态不应该与主句相同吗，为什么还用will

Answer 1

宾语从句的时态不是非得与主句一致。I don't know who will come. “我不知道谁会来”。从句用将来时是因为come 这个动作还未发生。

你可以仔细看一下下面有关宾语从句时态的问题。

宾语从句的时态
宾语从句的时态原则上应受主句的时态的影响。
1. 当主句是一般或现在类时态时 (如：一般现在时、一般将来时、现在进行时、现在完成时)，宾语从句可根据实际情况使用任何时态。如 He says that he often gives up. / He says that he has given up. / He says that he will give up. // He has told me that he always starts to work at 6:00. / He has told me that he started to work yesterday. / He has told me that he will start to work next week.
2. 当主句是过去类时态时(如：一般过去时、过去将来时、过去进行时、过去完成时)，宾语从句须用过去类时态。如 They said that they caught a cold. / They said that they had caught a cold. / They said that they would caught a cold.
3. 当宾语从句表示的是一个客观事实或真理时，无论主句是什么时态，从句一律用一般现在时。如 The teacher says ( will say / has said / said / had said ) that the earth moves around the sun.