There are some German nouns that do not take a plural ending.
They stay the same in singular – and in plural.
The context of a sentence will tell you whether one or multiple person(s) or object(s) are the topic of the conversation.

1. In this category we find nouns ending in -chen and -lein.
   Please note that these forms mark the German diminutive form. Example:
   das Kätzchen → die Kätzchen
   small cat → small cats
   das Kindlein → die Kindlein
   young child → young children
2. Nouns ending in -el, -er or -en stay the same in singular and plural.
   Please note that the vowel in the word’s stem has to change to mark the plural.
   The vowels -a, -o and -u change to -ä, -ö and -ü.
   Example:
   die Mutter → die Mütter
   mother→ mothers
   der Vater → die Väter
   father → fathers