I have just come back from USA,这句为什么不用came?

I have Just come back翻译成中文是什么哦? I just want come 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? -Did you work out the problem?yes,I --to the teacher’s为什么have just been,不能 have just come I'm a s____ in this town for I have just come here I just come back from i just come back form I have just come back from USA,这句为什么不用came? I have just come back from Tiawam ,the biggest of China.改为定语从句? I have come so just you want just i have 快I am not a doctor .I a helpingI am not a doctor.I a helping hand until the medical workers come A.am just lending B .have just lent C.just lent D.will just lend. I would have just said I have just finished it I have just been accustomed I just have one ___question I JUST HAVE MUCH TIME I have just come here to see if I can be to youhave just come here to see if I can be to youA.helpB.of helpC.with helpD.at help