More than you ever wanted to know about i-mutation -- or maybe not
So, let's talk about i-mutation. This is a sound change that took place in Proto-Germanic along the way to the various attested Germanic languages. The usual plural for many words ended in *-iz. So, *mannuz for "man," pl. *manniz. The A is an ah sound, the N is doubled, and the I in the plural is a long ee sound. If no i-mutation had occurred, this might have developed into a mod. Eng. "man, mans."
But flip those vowel sounds back and forth a few times: ah-ee, ah-ee. The A is formed in the back of the mouth, while the I is formed in the front. This is tiring. The fast and lazy way is to make the two operate out of the same region of the mouth. And so the first vowel is "fronted," that is, moved forward in the mouth. The plural then becomes *menniz. But since you now have the first syllable indicating that this is a plural, you don't need the second syllable, and -- once again, the fast and lazy way -- the second syllable drops off. Meanwhile, the ending dropped off the singular as well. Which is why OE mann has the plural menn, and we say the same: "man, men."
The same thing happened with geese and mice. Proto-Germanic *gans- eventually becomes OE gos (though the N stuck around in "gander"). The plural ending *-iz, however, changes the first vowel to E in the plural, and then you don't need the ending anymore to tell that it's more than one bird. So, gos, ges. Along comes the Great Vowel Shift later on, and the long O becomes an oo sound, while the long E (as in "fete") becomes a long I sound (as in "machine"). So mod. Eng. "goose, geese."
Now, OE mus ("mouse") is pronounced "moose," so why isn't its plural "meese?" Well, because the i-mutation of O in gos produced ges, but the i-mutation of mus produced pl. mys in OE. That Y was originally pronounced like a long Ü in German: an EE sound made with rounded lips. When the Great Vowel Shift came along, raising the position of all the long vowels in the mouth, the vowels at the top of the mouth couldn't go any higher, and they became diphthongs. So mus became "mouse." And the Y in mys became AI as in "high." So mod. Eng. "mouse, mice."
Okay, so why didn't the plural of "house" become "hice?" Because OE hus was neuter and had no inflection in the plural. Like mod. Eng. "sheep" and "deer," the plural of an hus ("one house") was just twegen hus ("twain house"). So hus never underwent i-mutation. But in Middle English, a lot of the old, funky plurals got regularized by adding -(e)s. OE scip ("ship"), e.g., took the plural scipu, but we say "ships" today. An (-es) got added to hus. And a little later, the Great Vowel Shift moved the OO sound in hus to an OU, so mod. Eng. "house," pl. "houses." (Note: not all dialects have really settled into the OU sound. Canadian English still says "the Scootmaster's aboot the hoose somewhere.")
But flip those vowel sounds back and forth a few times: ah-ee, ah-ee. The A is formed in the back of the mouth, while the I is formed in the front. This is tiring. The fast and lazy way is to make the two operate out of the same region of the mouth. And so the first vowel is "fronted," that is, moved forward in the mouth. The plural then becomes *menniz. But since you now have the first syllable indicating that this is a plural, you don't need the second syllable, and -- once again, the fast and lazy way -- the second syllable drops off. Meanwhile, the ending dropped off the singular as well. Which is why OE mann has the plural menn, and we say the same: "man, men."
The same thing happened with geese and mice. Proto-Germanic *gans- eventually becomes OE gos (though the N stuck around in "gander"). The plural ending *-iz, however, changes the first vowel to E in the plural, and then you don't need the ending anymore to tell that it's more than one bird. So, gos, ges. Along comes the Great Vowel Shift later on, and the long O becomes an oo sound, while the long E (as in "fete") becomes a long I sound (as in "machine"). So mod. Eng. "goose, geese."
Now, OE mus ("mouse") is pronounced "moose," so why isn't its plural "meese?" Well, because the i-mutation of O in gos produced ges, but the i-mutation of mus produced pl. mys in OE. That Y was originally pronounced like a long Ü in German: an EE sound made with rounded lips. When the Great Vowel Shift came along, raising the position of all the long vowels in the mouth, the vowels at the top of the mouth couldn't go any higher, and they became diphthongs. So mus became "mouse." And the Y in mys became AI as in "high." So mod. Eng. "mouse, mice."
Okay, so why didn't the plural of "house" become "hice?" Because OE hus was neuter and had no inflection in the plural. Like mod. Eng. "sheep" and "deer," the plural of an hus ("one house") was just twegen hus ("twain house"). So hus never underwent i-mutation. But in Middle English, a lot of the old, funky plurals got regularized by adding -(e)s. OE scip ("ship"), e.g., took the plural scipu, but we say "ships" today. An (-es) got added to hus. And a little later, the Great Vowel Shift moved the OO sound in hus to an OU, so mod. Eng. "house," pl. "houses." (Note: not all dialects have really settled into the OU sound. Canadian English still says "the Scootmaster's aboot the hoose somewhere.")