I have just come back.是现在完成时吗?如果是,那come为什么不用过去分词?

I have just come back.是现在完成时吗?如果是,那come为什么不用过去分词? there's none left.中left是leave的过去分词,那这一句是现在完成时吗?为什么不用加have/ha I have Just come back you have just come back your hometown为什么不用came I have just come back from USA,这句为什么不用came? I have no idea _when___he will come back.(whether可以吗?如果可以那if come的过去分词和现在是? 到底是I come back还是i am come back? I just come back from i just come back form I have been busy since yesterday.现在完成时是have加过去分词,但这里的过去分词在哪里 我回到了温哥华 是i have come back vancouver 还是i have come back to va