I have just bought a new bike 这里为什么用bought

I have just bought a new bike 这里为什么用bought i bought a new bike just now用just改写 i（ ）just bought a new bike just now.用just改写 I bought a new bike just now l.（用just改写） I I bought a new bike just now.(用just改写） who bought a new bike yesterday?疑问句中动词不是用原型嘛?为什么这里用bought而不是 I have just bought a new bike.这句话是对的,可是买是不可延续的啊, I have just bought a _____pencil box. I ___you have bought a new car. He have already bought a new bike改错 I hear that you have bought a new bike．Can you________it to i just have bought a car.和i have just bought a car.