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

你误解了
现在完成时不用瞬间动词，而应该用延续性动词只是在和for+时间段时才有这样的规定。
瞬间动词可以用完成时的，如过上面是这样的：I
have
just
bought
a
new
bike
for
a
year就不能这样用了。

扩展阅读：the blend 117 ... i have a book ... just have 中文翻译 ... don t have to ... 完型填空ihavejust ... in the soop seventeen ... have classes ... have a nap ... i have no idea ...