Why are some nouns declined like adjectives?

Question

It was noted in a previous question that der Beamte is declined like an adjective altho it's a noun. Is there a deeper reason to this? Do the nouns which are declined like adjectives share something in common? Thanks.

Answer 1

1. Adjectivic declension of nominalized adjectives

All nouns that derive from adjectives are declined in a very special way, because, although they are nouns (they have articles and are written with an uppercase first letter), the adjective still is alive inside this word and influences the grammatical behavior:

• adjective used as attribute of a noun
  The indefinite article that is used here to indicate the gender of the noun belongs to the noun, not to the adjective.

  masculine: ein schöner Löffel, ein schöner Mann
  feminine: eine schöne Gabel, eine schöne Frau
  neuter: ein schönes Messer, ein schönes Kind

• nominalized adjective
  The article still indicates the gender of the noun, but now the noun is a word that is derived from an adjective and so keeps many of the properties of the adjective

  masculine: ein Schöner
  feminine: eine Schöne
  neuter: ein Schönes

2. Origin of the noun »Beamter«

Noun: das Amt

The German noun »das Amt« means agent, department, office, so, an Amt is a sub-unit of the government. An Amt is never a private organization or part of it. Amt describes the logical unit or department. Sometimes Amt even means the whole government.

Das Amt der Steiermärkischen Landesregierung ist in rund 20 Dienststellen aufgeteilt, die an unterschiedlichen Standorten in der Stadt Graz untergebracht sind.
The Office of the State Government of Styria is divided into about 20 offices, which are located at different sites in the city of Graz.

The word Amt also describes the physical location of the authority, i.e. the whole building or that part of a building that is used by the authorities.

Im Standesamt Wien-Zentrum gibt es zwei Trauungssäle.
There are two wedding halls at the Vienna City Center Registry Office.

A third meaning of Amt is the power that is given from the authorities or from the people to a single person when this person works as employee of an official agency or department.

Es sind meistens Männer, die das Amt eines Bürgermeisters inne haben.
It is mostly men who hold the office of mayor.

Verb: jemanden beamten; Participle: jemand wird/ist beamtet

This verb is rarely used, and when its used, then it almost always is used in sentences using passive voice, so when it appears in a German sentence, it does so in form of a participle:

• Zustandspassiv (state passive)

  Herr Müller ist schon seit 30 Jahren beamtet.
  Mr. Müller has been appointed as a civil servant for 30 years.

• Vorgangspassiv (process passive)

  Markus wird bald beamtet werden.
  Markus will soon be appointed as a civil servant.

Adjective: beamtet

Participles are twilight beings, located in the intermediate world between verbs and adjectives. They are usually derived from verbs but can be used like adjectives, and many of them are used so rarely as verbs and so often as adjectives that they are counted entirely among adjectives. The word beamtet is such a word, and this is also the reason why neither the verb beamten nor the participle beamtet are listed in dictionaries. You only can find the adjective beamtet.

Wie viele beamtete Universitätslehrerlnnen gibt es? (source)
How many tenured university teachers are there?

Nominalized adjective: ein Beamteter, eine Beamtete

This is a normal nominalized adjective, as described in section 1 of my answer. Grammatically, it behaves like any other nominalized adjective. It is only used very rarely because there has been a shortened form of this nominalized adjective since the 14^th century:

Shortened nominalized adjective: ein Beamter, eine Beamte

This word is not a "real" noun, but still a nominalized adjective and therefore declined like a nominalized adjective:

• without article: strong declension

  masc.: Herbert ist Beamter.
  fem.: Sabine ist Beamte.
  (no neuter form, because only persons can be Beamter and there are no neuter persons.)

• definite article: weak declension

  masc.: Der Beamte heißt Huber.
  fem.: Die Beamte heißt Fink.

• indefinite article: mixed declension

  masc.: Ein Beamter arbeitet für ein Amt.
  fem.: Eine Beamte arbeitet für ein Amt.

But note, that the feminine form shown here is used very rarely, because there is an alternative feminine form:

Feminine noun: die Beamtin

This is a full noun, i.e. it's not a nominalized adjective, and therefore not declined like an adjective, but like every other noun:

Sabine ist Beamtin.
Die Beamtin heißt Fink.
Eine Beamtin arbeitet für ein Amt.

Answer 2

Because they are nominalized adjectives who got another use as a common term for a thing. This happens very very often. Those nouns do not have anything else in common.

• der/die/das Neue
• der/die/das Fremde
• das Gute
• das Böse
• der/die Blinde
• der/die Kriminelle
• der/die Kranke
• der/die Obdachlose
• der/die Deutsche
• …

And it happens to participles as well (as they are like adjectives):

• der/die Reisende
• der/die Überlebende
• der/die Betrunkene
• der/die Bekannte

And even to gerundives:

• der/die Auszubildende