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'Viren' and 'Rhythmen': when '-us' and '-os' give way to '-en'

Some German nouns borrowed from Latin and Greek form their plural in a surprising way. Instead of adding something to the end of the word, German cuts off the ending -us (or -os) and puts -en in its place: der Rhythmus becomes die Rhythmen, das Virus becomes die Viren and der Mythos becomes die Mythen.

English speakers find this tricky because English simply adds -es to the whole word: virus, viruses; rhythm, rhythms. So the natural instinct is to say Virusse or Rhythmusse. Those forms sound wrong to German ears. The good news is that the pattern is very regular once you know which words belong to it.

Word list
ComputervirusB1 virus das Computervirus, die Computerviren KubusmB1 cube der Kubus, die Kuben RhythmusmB1 rhythm der Rhythmus, die Rhythmen VirusB1 virus das Virus, die Viren KaktusmB2 cactus der Kaktus, die Kakteen (also Kaktusse) MythosmB2 myth der Mythos, die Mythen OrganismusmB2 organism der Organismus, die Organismen RassismusmB2 racism der Rassismus, die Rassismen (forms of racism) SarkasmusmB2 sarcasm der Sarkasmus, die Sarkasmen ExorzismusmC2 exorcism der Exorzismus, die Exorzismen FaschismusmC2 fascism der Faschismus, die Faschismen (die europäischen Faschismen) MetabolismusmC2 metabolism der Metabolismus, die Metabolismen NationalismusmC2 nationalism der Nationalismus, die Nationalismen (die Nationalismen Europas) RhombusmC2 rhombus der Rhombus, die Rhomben ZynismusmC2 cynicism der Zynismus, die Zynismen (cynical remarks), like Sarkasmen
How it works

Take the singular, remove the final -us or -os and add -en. Nothing else changes, and the article in the plural is always die, as for every German plural.

  • der Rhythmus → Rhythm- → die Rhythmen
  • das Virus → Vir- → die Viren
  • der Mythos → Myth- → die Mythen

This is a feature of a particular group of learned words, not a rule for every noun ending in -us. A useful hint: words ending in -ismus almost always follow it. Think of Organismus, Mechanismus or Algorithmus, which become Organismen, Mechanismen and Algorithmen. Zyklus (cycle) also becomes Zyklen.

The words
Everyday words

Virus and its compound Computervirus are the ones you will meet most often. Both are usually neuter in standard German. Rhythmus comes up in music, dance and talk about daily routines.

Im Winter verbreiten sich Viren besonders schnell.
(In winter, viruses spread especially quickly.)
Das Programm erkennt verschiedene Computerviren.
(The program detects various computer viruses.)
Wir üben heute zwei neue Rhythmen.
(We are practicing two new rhythms today.)
Shapes

In geometry, Kubus (cube) and Rhombus (rhombus) follow the same pattern. In everyday speech people often say Würfel instead of Kubus.

Die Künstlerin zeigt große Kuben aus Glas.
(The artist is showing large glass cubes.)
Auf dem Arbeitsblatt sind zwei Rhomben und ein Quadrat.
(There are two rhombuses and a square on the worksheet.)
Stories, life and science

Mythos ends in -os rather than -us, but it works exactly the same way. Organismus and Metabolismus belong to biology.

Viele alte Mythen erzählen von Göttern und Helden.
(Many old myths tell of gods and heroes.)
In einem Tropfen Wasser leben unzählige winzige Organismen.
(Countless tiny organisms live in a drop of water.)
Die Forscher vergleichen die Metabolismen verschiedener Tierarten.
(The researchers are comparing the metabolisms of different animal species.)
Other -ismus words

Sarkasmus and Exorzismus are mostly used in the singular. When you do need a plural, it is formed in the usual way: Sarkasmen means sarcastic remarks.

Seine ständigen Sarkasmen gehen mir auf die Nerven.
(His constant sarcastic remarks get on my nerves.)
Der Film zeigt mehrere Exorzismen in einem alten Kloster.
(The film shows several exorcisms in an old monastery.)
Watch out

The typical mistake is to keep the whole singular and add a plural ending, as in Virusse, Rhythmusse or Organismusse. Remember that the -us goes away completely.

Kaktus is a slightly different case. Its standard plural is Kakteen, with a double e, and the regular-looking Kaktusse is also common in everyday speech.

Auf meiner Fensterbank stehen drei Kakteen.
(There are three cacti on my windowsill.)

Many other -us nouns do not follow the pattern at all: der Bus becomes die Busse, der Zirkus becomes die Zirkusse and der Bonus becomes die Boni or die Bonusse. Atlas has its own special plural, Atlanten. When you are unsure, check the plural in the dictionary.

Summary

For learned words such as Virus, Rhythmus, Mythos and nearly every word ending in -ismus, form the plural by replacing -us or -os with -en: Viren, Rhythmen, Mythen, Organismen. Never add an ending to the full singular, and keep in mind that ordinary words like Bus and Zirkus simply take -se.