I understand what is the meaning of 别说了. What is 快 doing? According to Google Translate: 快别说了 as “Stop talking”. 别说了 as “do not talk”.
Does the 快 change the tone of 别说了? If so, how do you translate differently from Google?
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快 (quick; hurry) here is a [command/ demand exclamation] We use it in front of a verb or verb phrase to stress the urgency, demands immediate compliance.
(a request) 别说了 = "you've said enough" or "stop talking" (no tone indicated)
(a demand) 快别说了 = shut up (快住口) or 'Stop talking! Now!'
It is similar to 'Now!' in English
Drop your weapon. (no tone indicated)
Drop your weapon! Now! (commanding/ urgent tone)
快住手 = hurry and stop = Stop! Now!
快去救人 = hurry and go save people = Go save people! Now
快 can also be used as the short form of '就快' (soon) or '快要' (almost)
(就)快沒彈藥了 - out of ammunition soon
(就)快餓死了 - starving to death soon
快(要)沒彈藥了 - almost out of ammunition
快(要)餓死了 - almost (about to) starve to death
别说了 - is a general statement of "Stop it (talking), I don't want to hear/discuss it".
快别说了 - usually has a sub/hidden-clause attached to it, "Stop it (talking), because.... (a reason or concern that makes the talking inappropriate that may lead to adverse consequence)", such as 快别说了, 别惹麻煩.
So the difference is the urgency, obviously the latter is more critical than the former. But there is nothing wrong if you say 别说了, 别惹麻煩 (a general statement).