Imperfect verses pe compose?

can some one explain when to use each in simple terms because i have when to use it but I'm just not getting it.I have this whole entire packet to due and im not sure if im doing it right. ex: Jerome a telephone a ses parents car Il alliat etre en retard. or another ex: tu a achete une patisserie car tu etais faim.. are these right? if not could you correct them. wow 5 years of french and this is what I get..