What is the rule for adding -n in the dativ plural case?

Question

I have the feeling that this is correct:

mit guten Ergebnissen

and this is not correct:

mit guten Ergebnisse

But I don't really know a rule here for other nouns, e.g. "mit guten Bäumen"? or "mit guten Monitoren"?.

Do you always add a "-n" to all plural nouns that are in the dativ case?

Answer 1

The rule is: if you use dative plural you have to append an -n to the nominative plural form:

• die Monate / den Monaten
• die Eier / den Eiern
• die Bäume / den Bäumen

There are two exceptions to this rule:

1. If the nominative plural already ends with an -n, it remains the same:

   • die Eltern / den Eltern
   • die Hunnen / den Hunnen
   • die Mädchen / den Mädchen

2. If the plural form ends with an -s:

   • die Fotos / den Fotos
   • die Ufos / den Ufos
   • PCs / den PCs

Answer 2

The rule is very simple: the dative plural of nouns almost always ends in "-n".
There are very few exceptions like:

ich gebe es den Opas, den Omas.

Also, a few words of English origin have their dative plural in "-s" :

Aus den Parks, aus den Wracks, auf den Decks.

But these exceptions are statistically negligible. The robust rule is:

Use "-n" for nouns in the dative plural.

Answer 3

In addition to those exceptions mentioned before (nouns with -n and -s ending in plural) you shouldn't add an -n in dative in most nouns with non-native plural endings (not only English/French "s", but also Latin or Greek endings). E.g. den Tempora (plural of Tempus - "tense"), den Modi (plural of Modus - "mode"), den Praktika (plural of Praktikum - "internship"). All of those words aren't used very often, though.