I can’t listen to stupid people.
Is there any particular reason why this is dummen and not dumme?
I normally use this flowchart to understand adjective endings -
but as I read it, this would mean that the adjective should be dumme.
“dummen Menschen” is dative plural (because zuhören takes a dative object), so Question 3/NO of that handout applies here, or maybe already Q2/NO?
The handout is a bit weirdly organized. It says when there’s no article then you use the “der-word ending”. I’m not even sure what that is. I mean Question 1/NO has an example where the adjective gets an -en (“kalten Kaffee”), but that example is accusative singular. The handout doesn’t talk much about cases, but obviously they are important. Q2 mentions “standard, unchanged form”; that seems to ask whether it’s nominative or one of the other cases.
You can add an article to that sentence, so the dative plural becomes a bit clearer:
Ich kann den dummen Menschen nicht zuhören.
That changes the meaning just like in english (from stupid people in general to a specific known group), but with or without article, the adjective gets an -en ending (because of dative plural).
It is “dumme Menschen” only in nominative and accusative without article. In all other cases it’s “dummen Menschen”:
nom: Dumme Menschen treffen dumme Entscheidungen.
gen: Dummen Menschen Rede macht oft keinen Sinn. (somewhat archaic german, edit: also wrong - see post below)
dat: Dummen Menschen kann ich nicht zuhören.
acc: Dumme Menschen mag ich nicht.
nom: Die dummen Menschen treffen dumme Entscheidungen.
gen: Der dummen Menschen Rede macht oft keinen Sinn. (same)
dat: Den dummen Menschen kann ich nicht zuhören.
acc: Die dummen Menschen mag ich nicht.
Thanks so much for that wonderful explanation.
I had missed the fact that zuhören takes a dative object, and so that would have been enough for me to understand (but the rest of this explanation is very helpful).
In your example sentences, I love the way that you have put the various forms of (die) dumme Menschen at the beginning of those sentences to make things clear. That is very helpful indeed (and a nice set of examples too).
In the handout, I understand the term “der-word ending” to mean whichever der/die/das/den/des/dem form the sentence would use if it were the definite article instead of the adjective. In other words, I read that as meaning “add the ending that would occur on a der-word appropriately declined definite article for that noun”. I wasn’t entirely sure at first (e.g. it could have been suggesting to use the declined masculine forms, but the Dieses Bier example cleared that up for me).
Then, for “standard, unchanged form” I take that to mean the nominative form, as you mentioned.
I have found using this flowchart to be helpful, much more helpful than trying to remember the one and a half pages of tables in my old grammar book 
.
I now just need to pay more attention to whether there are any changes of case due to a verb sitting at the end of the sentence.
I mean, I don’t want you thinking “Was für ein dummer Mensch hat diese dumme Frage gestellt?” 
Thanks for your help.
P.S. I had just spent a few minutes staring at the sentence Spielt ihr Jungs in derselben Mannschaft? and wondering why derselben didn’t have a dative ending, until I suddenly realised that the “der” bit of derselben had already declined from the “die” bit of dieselbe. I should probably have taken that as a signal to take a break and make a cup of tea.
I’m starting to understand how the handout works.
It is really Q1/NO that applies here, and you would take the -en suffix from “diesen dummen Menschen”. It should really call it the “dieser-ending”, like they used in the examples, because a normal “der/die/das” definite article does not decline in a way where you can take the suffix directly from it. That said, if you’re trying to parse a sentence, and not construct one, then if the sentence does not feature an article, it is of course hard to see where the ending comes from when you’re not aware of the case the adjective and noun are in.
Also, they use the term “standard unchanged form” because it’s not the same as the nominative, as you can see from my accusative-with-article example, where the accusative looks identical to the nominative.
I mean, I don’t want you thinking “Was für ein dummer Mensch hat diese dumme Frage gestellt? ”
Don’t worry! 
I just noticed I made a mistake. The genitive-without-article example should read:
Dummer Menschen Rede macht oft keinen Sinn.
The genitive case, especially when used without an article, and putting it before the noun, has gotten so out of fashion, that it’s easy to forget how that works… 
In modern german, people would never say it like that. Maybe instead:
Was dumme Menschen sagen macht oft keinen Sinn.
Or people would replace the genitive with “von”+dative:
Das Gerede von dummen Menschen macht oft keinen Sinn.