為何英文字最後字母是 o 時眾數要加 es ?為何有些例外?

Well.....unfortunately in English, there is no rule that isn't broken ....you know what .....there are a lot of exceptions don't have any reasoning behind them.....
I guess ......this would be one of those freaky exceptions....

The plurals of words ending in "o" are formed by either adding "s" or "es" .... some of the words can be formed either way...
To determine whether a particular word ends in "s" or "es" [or if the word can be spelled either way]......my suggestion to you is check your dictionary. ^_^

Well..there are two rules you need to remember.......

1)All the words that end in a vowel ("ao", "eo", "io", "oo", "uo") have plurals that end in just "s"......
For example:
studio------> studios
duo ------> duos
zoo ------> zoos

2) All musical terms ending in "o" have plurals ending in just "s"....
For example:
piano------> pianos
cello------> cellos
solo------> solos

Hope it helps ~ ^______^

2009-03-02 16:04:17 補充：
Ahhhhhh.....I remembered now ...for the "oes" rule......when most nouns ending in "o" preceded by a consonant ....then we need to add "es".....

p.s. Consonant ---> "B,C,D,F,G,H....etc." [doesn't include "a,e,i,o,u" ]

2009-03-02 16:04:31 補充：
For example:
1) potato -----> potatoes [ since "t" is a consonant]
2) tomato -----> tomatoes [ since "t" is a consonant]

2009-03-02 16:04:38 補充：
3) buffalo -----> buffaloes [ since "l" is a consonant]
4) mango -----> mangos or mangoes [this case is different....they can both be used] <--- don't ask me why.....as I said before "some of the words can be formed either way" ^_______^