While most English words are made plural by adding an "s" or "es" to the end of the word, English also has many irregular nouns, which form plurals in a variety of ways. Linguists sometimes call these “mutating plurals.” While some irregular nouns are the same whether they are singular or plural, others keep their original foreign plurals. Some other irregular nouns do change their vowel sounds to form the plural, as well.
No Other Change
A very few irregular nouns alter only their vowel sound, with no other changes. “Man” becomes the plural “men,” for instance, and “foot” becomes “feet.” There is no general rule here; “goose” becomes “geese,” but other double-o nouns, like “moose” and “roof,” do not change in this way. One category of nouns which do follow a rule is those words ending in “is” that derive from Latin; the ending changes to “es.” Thus, “oasis” becomes “oases,” and “crisis” becomes “crises.”
Additional Changes
Some irregular nouns change vowel sounds in addition to other changes to the word. On rare occasions, the vowel itself doesn’t change, only its sound, as when “child” becomes “children.” Some common nouns get shorter, not longer, as when “mouse” becomes the plural “mice.” Again, there is no convenient rule these nouns follow; “house,” for instance, is a regular noun, although it looks just like the irregular “mouse.”