Which one to choose: 赶紧 or 快? What's the difference between those two? For instance, in a sentence like "你们赶紧/快走吧" can I use both indiscriminately? What are the particular conditions that make me prefer one rather than the other?
In the usage of the sentence “你们赶紧/快走吧”

Yes you can use both indiscriminately.

The only different between “赶紧” and “快” in the sentence above is the number of words.


赶紧 means you have to do it now
快 got other usage but one of them is same as 赶紧


In my opinion, if I have to use 快 in the above sentence it would be like I'm shouting 快走吧！！ shorten the number of sounds I have to pronounce.

Then I will use 赶紧 in the above sentence it would be like I'm talking gently 你们赶紧走吧