Declension: der/die Kranke (sick person)—why "Kranken" in "Alle Kranken brauchen Ruhe"?

Question

The following sentence from a course quiz of mine deals with the adjectival noun "der/die Kranke" (sick person):

Alle [...] (Krank-) brauchen Ruhe.

Here is how I approached this:

• There is no article, meaning the adjectival noun must assume a strong ending.

• The case here is the nominative plural.

• Therefore, since the strong ending for nominative plural is -e, I answered:

Alle Kranke brauchen Ruhe.

However, my instructor says this needs to be:

Alle Kranken brauchen Ruhe.

Could someone please help me understand why this noun is declined like this? I thought perhaps der Kranke is N-declined, but that doesn't seem to be the case from my research.

The only other explanation I can come up with is that this is a mixed ending and not strong, but "alle" is not an indefinite article like "ein/kein/mein/sein/etc."

Answer 1

You state:

There is no article, meaning the adjectival noun must assume a strong ending.

This is a correct rule of thumb, however alle does count as article. [1] The same goes for pronouns. Thus using the interrogative »welcher«, one would say »Welche Kranken brauchen Ruhe?«, that is, have weak declension. (Strictly speaking the rule applies only to articles "mit Endung". You'd say »Solcherlei Kranke brauchen keine Ruhe« because the article »solcherlei« is "endungslos".)

As an aside, there are exceptions to the rule. The grammar issued by Duden says:

Ebenso werden nach Adjektiven, die demonstrativen Artikelwörtern und dem Indefinitum alle nahekommen, die darauffolgenden Adjektive tendenziell wie nach starken Artikelwörtern flektiert, also schwach. (8th edition, p. 952)

That is, if you have an adjective (or adjectival noun, for that matter) that is preceded by an adjective similar to demonstrative articles or "alle", then that adjective typically is weakly declined. One of their examples is »folgende wichtigen Meldungen« (because "folgende" is demonstrative-like).

[1] In German grammar such words are called "Artikelwort". They are not exactly articles, but words that may be used like articles.

Answer 2

Alle is not an adjective but behaves like an article. Because of this, the following adjectives are weakly inflected.

The same applies for pronouns like dieser/diese/dieses:

Diese Kranken brauchen Ruhe.