just now的意思是“刚才”,所以是过去时
just意思是刚刚好,举个例句吧
I have just finished my homework
意思就是我刚刚完成我的作业
just then 就在那时
Just then the meaning of what had been said burst upon him.
就在那个时候,他突然领悟到刚才那番话的含意.
It was just then that he heared the cry in the grass.
就在这个时候,他听到了草丛里的叫声.
just now
1）如果用于过去式,意为“刚才”；
I saw him just now.
我刚才见过他.
The foreign guests arrived just now.
外宾刚到.
2）如果用于现在时,表示“now”的强调,意为“现在,此刻”；
I am busy just now.
我此刻很忙.（此句比I am busy now的语气强）