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'Museen', 'Praktika', 'Daten': plurals that replace the ending '-um'

Many German nouns borrowed from Latin end in -um: Museum, Zentrum, Album, Praktikum, Datum. In the plural these words do not add an ending to the whole word. Instead, the -um is removed and a new ending takes its place: Museum becomes Museen, and Praktikum becomes Praktika.

English speakers trip over this for two reasons. English usually just adds -s (museums, albums, centers), so Museums or Albums feels natural. German plural habits also suggest adding -e or -en to the full word, which gives impossible forms like Museumen or Zentrume. Neither works: the -um has to go.

Word list
DatumnA1 date die Daten MuseumnA1 museum die Museen AlbumnA2 album die Alben EinkaufszentrumnA2 mall die Einkaufszentren UniversumnA2 universe die Universen ZentrumnA2 center die Zentren ForumnB1 forum die Foren GymnasiumnB1 secondary school die Gymnasien LaboratoriumnB1 laboratory die Laboratorien MysteriumnB1 mystery die Mysterien StadtzentrumnB1 downtown die Stadtzentren VisumnB1 visa die Visa AntibiotikumnB2 antibiotic die Antibiotika AquariumnB2 aquarium die Aquarien GeburtsdatumnB2 date of birth die Geburtsdaten ImperiumnB2 empire die Imperien IndividuumnB2 individual die Individuen MillenniumnB2 millennium die Millennien AblaufdatumnC1 expiration date die Ablaufdaten FaktumnC1 fact die Fakten MinisteriumnC1 ministry die Ministerien StipendiumnC1 scholarship die Stipendien VerfallsdatumnC1 expiration date die Verfallsdaten KuriosumnC2 curiosity die Kuriosa PraktikumnC2 internship die Praktika ReferendumnC2 referendum die Referenden VakuumnC2 vacuum die Vakua
How it works

Take the singular, cut off -um, then add the plural ending. There are two possible outcomes:

-um → -en

The usual pattern. Zentrum becomes Zentren and Album becomes Alben. If a vowel comes right before -um, it stays: Museum becomes Muse-en, which is written Museen, and Aquarium becomes Aquarien.

-um → -a

The original Latin plural, kept in a smaller group. Praktikum becomes Praktika and Visum becomes Visa.

All of these nouns are neuter (das Museum), so the plural article is simply die. A useful bonus: these plurals already end in -en or -a, so you never add an extra -n in the dative. You say in den Museen and mit den Praktika.

In der Stadt gibt es drei Museen.
(There are three museums in the city.)
The words
Everyday words with -en

These are worth learning first: Datum → Daten, Album → Alben, and the Zentrum family: Zentrum, Einkaufszentrum and Stadtzentrum → Zentren, Einkaufszentren, Stadtzentren. Compounds follow their last part, so every word built on Datum works the same way: Geburtsdatum, Ablaufdatum and Verfallsdatum all end in -daten.

Welche Daten passen dir für das Treffen?
(Which dates work for you for the meeting?)
Bitte überprüfen Sie die Geburtsdaten der Teilnehmer.
(Please check the participants' dates of birth.)
Bei Milch und Käse sollte man immer die Verfallsdaten prüfen.
(With milk and cheese you should always check the expiration dates.)
Meine Eltern haben viele alte Alben mit Fotos.
(My parents have lots of old photo albums.)
In der Nähe gibt es zwei große Einkaufszentren.
(There are two big malls nearby.)
Am Sonntag sind viele Stadtzentren ziemlich ruhig.
(On Sundays many downtown areas are pretty quiet.)
Words ending in -ium

Here the i stays and -en is added: Aquarium → Aquarien, Ministerium → Ministerien, Stipendium → Stipendien, Laboratorium → Laboratorien, Imperium → Imperien, Millennium → Millennien, Mysterium → Mysterien. The classic Studium → Studien and Gremium → Gremien work the same way.

Im Zoo gibt es mehrere Aquarien mit bunten Fischen.
(At the zoo there are several aquariums with colorful fish.)
Zwei Ministerien arbeiten an dem neuen Gesetz.
(Two ministries are working on the new law.)
Viele Studenten bewerben sich um Stipendien.
(Many students apply for scholarships.)
Other words with -en

Forum → Foren, Universum → Universen, Individuum → Individuen, Referendum → Referenden, Faktum → Fakten.

In diesen Foren diskutieren Leute über Computerspiele.
(In these forums people discuss computer games.)
Words with -a

Praktikum → Praktika, Visum → Visa, Kuriosum → Kuriosa, Vakuum → Vakua.

Sie hat schon drei Praktika gemacht.
(She has already done three internships.)
Für die Reise brauchen wir zwei Visa.
(We need two visas for the trip.)
Watch out
  • The typical mistake is keeping the full singular: Museums, Zentrums and Praktikums are wrong as plurals. These forms do exist, but only as the genitive singular: des Museums means of the museum.
  • Daten means two things. Daten is both the plural of Datum (dates) and the everyday word for data. Context decides: Daten speichern means to save data.
  • Some words have two plurals. Visum has Visa or Visen. Vakuum has Vakua or Vakuen. You will also see Millennia, Referenda and Fakta. The forms listed above are the safe choice.
  • Fakten is very common, but people usually treat it as the plural of the shorter word Fakt. Faktum itself sounds formal.
  • Other foreign endings behave in a similar way, for example Thema → Themen and Virus → Viren. So if a foreign-looking word seems to resist a normal plural, check its dictionary entry.
Summary

For foreign neuter nouns ending in -um, remove -um before forming the plural. Most take -en (Museen, Zentren, Daten, Ministerien), while a few keep the Latin -a (Praktika, Visa). Never add an ending to the full word: Museums is a genitive, not a plural.