Tigerman, that's a concept that's known as "umlauting". It's something that's common in Germanic languages. Very roughly, there are some nouns that will change one of their vowels when you pluralise them. Usually 'a' will change to 'æ', 'o' to 'ø', and 'u' to 'y':
- en mand, to mænd - one man, two men
- en bog, to bøger - one book, two books
- en ko, to køer - one cow, two cows
But you can also find umlauting in other grammatical situations, like when you form the comparative of an adjective:
- lang, længere - long, longer
- ung, yngre - young, younger
English, which is also a Germanic language, has some remnants of umlauting, like in the words "man" and "woman", which change their 'a' to 'e' in the plural form: "two men", "two women".