Norman Paxton : German Grammar
1. an umlaut can only be added to a,o or u, NEVER to e or i.
2. Feminine nouns NEVER change or add any endings in the singular forms.
3. Except for plural in -s ( as with das Hotel ), all nouns in German add -n to form Dative plural, unless the plural already ends in -n.
4. In the plural, the nominative, accusative and genitive forms of all nouns remain the SAME.