Reasons for non-inflection of adjectives are
- Fixed expressions (ruhig Blut)
- Color words (lila Haus)
- derivations with -er (Hamburger Marktplatz)
But "prima" does not seem to fit into any of those categories.
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Sign up to join this communityReasons for non-inflection of adjectives are
But "prima" does not seem to fit into any of those categories.
The rule, that color words are not inflected is wrong. Here are counter examples:
The reason, why »lila« and »rosa« are not inflected when they are used as attributes of nouns is a different reason:
Adjectives ending with »a« are very rare in German language. None of them is inflected:
In colloquial speech I found two more. They are adjectives that in good-style standard German are only used predicative, i.e. not as attribute of a noun:
Very likely the main reason is: Inflection would simply sound awkward to German speakers.
Non-inflectable adjectives often end with a vocal (like "rosa", "lila", "prima". Inflecting them makes them sound awkwardly stuttering.
(BTW: From my list of examples of indeclinable adjectives you can easily see that the three reasons you list in your question are by far not exhaustive)