"Passer de bons moments" ou "Passer un bon moment"
I saw "Passer de bons moments" in my textbook so it should be correct, but I just don't know why we use "de" here.

If "to have a good time" is translated as "Passer un bon moment", then "we had/ spent some good times" should be "Nous passions DES bons moments" instead of "de" ?

Question

"Passer de bons moments" ou "Passer un bon moment"
I saw "Passer de bons moments" in my textbook so it should be correct, but I just don't know why we use "de" here.

If "to have a good time" is translated as "Passer un bon moment", then "we had/ spent some good times" should be "Nous passions DES bons moments" instead of "de" ?

Answer

Il y a, en français, une règle qui dit que l'article "des" devient "de", si l'adjectif est placé devant le nom, et non derrière (la règle générale étant que l'adjectif vient après le nom). On dira par exemple : "nous avons mangé d'excellents gâteaux" et "nous avons mangé des gâteaux excellents".

Answer

Salut ! 
It's actually correct :) 
Here "de" is like "some" as you said so it works.
But if you say "des bons moments", nobody will even notice ahah

Answer

salut ! Here you can't use "des" because of the adjective. When you have an adjective before the noun, you have to write "de". 
So you can say : Je passe des moments agréables. But you say : Je passe de bons moments. It's just about the place of the adjective !