okay, In listening and speaking the difference is that: ز is the sound of Z in English ظ is like the sound of Th in English but the deep sound like in the word "Thus" so it's clear when to write them as you listen and speak

It depends on the word ..there is no certain rule for this & as the members mentioned before ز has a Z sound.. ظ has a deep Th sound but i didn't get what do you mean by (dza) ?? If it's referring to the letter ذ that one has a soft Th sound similar to the word (therefore) goodluck :)

If you mean by when to write them, well, the difference in sounds is clear, the ز is like the Z sound, and the ظ is like a deep Th sound. Otherwise, I'll appreciate if you can explain what do you mean by your question.

It depends on the word you're using my dear, could you be more specific? So me and the members could help you Farrah :)