If Adam and Eve didn`t know right from wrong before the fall, then how is it just that god punished them?

God told them not to eat the fruit of this tree.

You assume they didn't know it was wrong to sin against God before they did. What they did was partake of the fruit of the tree of THE KNOWLEDGE OF GOOD AND EVIL. They learned of good and evil. Up to that point, they did not know evil. They understood evil once their eyes were opened and hid from God rather than confess what they had done. Also, to say that they didn't know they were wrong until after the fact is wrong. They knew it was wrong before they did it because God TOLD them not to do it. If there's a law that says you can't drive 100 miles an hour in a school zone, and you know about it because you were told by the sign on the street, then if you do it and you get caught, you can't say you're innocent because you didn't know it was wrong until the cop told you it was.

God told them not to eat from the tree. To eat from it is disobedience. They had to have known that it was wrong to disobey because God told them that it would result in their deaths. Not physical death, but spiritual death.

Eve was deceived. She changed her interest in obeying God, to disobeying God because she focused on the fruit being nothing but good.. and the evil part was ignored.

The punishment was not directly coming from God, anymore than a child getting burned from touching a hot stove burner isn't directly punished by the parent who warned the child not to touch it.

because the choice wasn't contingent upon their knowledge of good and evil, but the expressed command from God. it is the difference in knowing it is good to eat from these trees, but forbidden to eat from this one tree... no knowledge of good and evil necessary to determine right and wrong. right was to obey God, wrong was to disobey. now just because they did not understand the consequences of the choice does not mean God is unjust in punishing them for sin. God had to punish sin, the worst thing for us and God is to live forever in His garden with no knowledge of our evil proclivities. again, the knowledge wasn't required to obey the word of the Lord. just like God doesn't owe anyone an explanation, yet provides one in the Bible.

The only thing such inquiries do is stimulate one to find more complex answers to that whole story. And when one does some research one learns many things like not to see it as it is written if one is to understand the specifics of what really happened.

They 'knew' in the sense of 'understood'. To 'know' in King James English means to 'experience'. As in Adam knew Eve.......

God warned them.

There was only one rule and they disobeyed it.