为什么相同的字母、字母组合要使用不同的音标？

1.元音:

1) [i:] 字母组合:ee ea e ie

three tree green sheep meet beef see seek

eat tea meat leave lead teacher team mean speak clean please

he she me

piece receive ceiling

2) 发音字母 i y e

sit picture it is list six mix fix fit pig big build miss

myth many twenty happy dictionary

defect decide delicious

3) [] 发音字母 a

bag hand and ant happy hat map mad bad black back glad flag shall man

4) [e] 字母组合 ea e a

head bread pleasure

elephant electric remember sell shell lesson better bed desk hotel yes

many any

5) [ε:] 字母组合ir ur ear er or

girl shirt skirt thirty thirteen third bird

turn burn murder nurse turtle Thursday burger

learn earn earth heard

term her nerd serve

work worm work world

6) [ε] 字母组合 er or ou ar o a e u

teacher le