例如下面的句子:while two detectives were keeping guard at the door.two other detectives opened the parcel,前面半句用了BE动词,与KEEP购进过去进行时,后面半句为什么不用BE动词?有没有一个规则定议,在什么情况下有实义动词时前面要加BE动词,什么时候又不用?