after，before，if，when，while，as soon as
(1)Never get on or off a bus ____ it is still running.
(2)____ X=Y and Y=Z，then X=Z.
(3)All things are difficult ____ they are easy.
(4)i'll talk about it with you ____ we meet next week.
(5)____ I left school at the age of seventeen，I went to college.
(6)____ the thief saw the policeman，he ran away as fast as he could.