1Sorry,I'm late.The car (broke down) on my way here.强调on the way here 途中the car broke down的,属于过去某个时间的动作,当时对我我造成的影响,而现在已经顺利达到目的地了,所以要用一般过去时；
2,The lift (has broken down).强调lift broke down这个动作对现在的影响,即到现在还没修好,我只好walk upstairs.如果用一般过去时,则只说明过去某个时间the lift broke down,但现在有可能已经修复,显然与 have to walk upstairs 相违背.