Quote Originally Posted by Galician View Post
Uhhh, wouldn't "man" and "men" fall into this declension pattern?
Good guess, but nope! This change is due to umlaut (Any German students ay be confused since the dots over ä, ö, ü are also called umlauts). To very much under-describe it, umlaut is vowel change. In German and English (and maybe other languages) it is used to mark some plurals: Man -> men. Mann -> Männer ( pronounced: Menn-er or Menn-uh); Mouse -> Mice. Maus -> Mäuse (pronounced: Moy-suh).

But brethren is the former plural of brother. It's rather archaic today though.