关于be about to...

解析： was/were about to do...when意思为“正要/正准备做某事，就在这时……”。再如：We were about to set out when it began to rain.我们正要动身，就在这时开始下雨了。句意：我正准备出去，这时一位不速之客来拜访我。而be going to 是将来式，而when引导的从句是过去式，所以不能和when用在一起。明白了吗？