请帮忙解答 He was about to rob the bank when he saw a police-car coming

be about to do something when 是固定搭配，意为正准备做某事，主从句都为过去式，see somebody \ something doing 意为看见某人正在做某事

see sb./sth. doing
看见某人/某物正在做……

从句是一般过去时。

see sth. doing：看见某物“正在” 。

这句话里的后半句when he saw a ploice-car coming实际上是个从句，等同于是when he saw that a ploice-car is coming。这样你就好理解了！多读，多背，有了语感，这根本就不是问题！