Umlaut in the plural number

DA7

Certain nouns form the plural number with the addition of umlaut to their stem vowel. This is the case with types A, B and C of plural number formation.

Types of plural		Singular	Plural with umlaut

Type A	ending -e
		Stadt	Städte
		Bach	Bäche
		Knopf	Knöpfe
		Ball	Bälle

Type B	ending -er
		Haus	Häuser
		Irrtum	Irrtümer
		Bad	Bäder
		Mann	Männer

Type C	without ending
		Vater	Väter
		Boden	Böden
		Vogel	Vögel
		Mutter	Mütter