i didn't know you had called the police before l came here

中文“我不知道……”后面只要跟的是陈述一件事,就要说 I didn't know...,因为跟在后面的事你不知道是过去,现在一定知道才会说.例如：
我不知道你已来了.
I didn't know you are here.
如果后面跟疑问句,表示现在仍不知,用 I don't know...,例如：
我不知道他几点走的.
I don't know what time he left.