In German dictionaries and vocabulary lists, singular nouns are listed along with a notation that indicates how to form the plural. There are five main patterns for forming plural nouns. (see table for examples)
Most masculine and neutral nouns form the plural by adding -e or -er. Plurals with the -er ending always add an Umlaut when the vowel in the singular form is an a, o or u.
der Tag (day) ---> die Tage
das Buch ---> die Bücher
If the singular form of a noun ends in -el, -en or -er, there is no additional plural ending, but an Umlaut is added to the stem vowel a, o or u.
der Apfel (apple) ---> die Äpfel
das Zimmer (room) ---> die Zimmer
For feminine nouns ending with -in, add -nen to form the plural.
die Freundin ---> die Freundinnen
die Studentin ---> die Studentinnen
For most other feminine nouns, add -n if the singular form ends in -e, -el or -er. Add -en if it does not. Note that feminine plurals with these endings never add an Umlaut.
die Blume (flower)---> die Blumen
die Frau ---> die Frauen
Plurals: Borrowed Words
The -s ending is added to most words borrowed from other languages and to most nouns ending with vowels other than e.
das Sofa ---> die Sofas
das Auto ---> die Autos
das Baby ---> das Babys