在26个英文字母中，a，e，i，o，f，h，l，m，n，r，s，x等12个字母的读音是以元音开头的，因此在这些字母单独使用时，前面用an.其余字母则是以辅音开头的，前面用a.
看底下字母的读音音标就一目了然：
A a [ei] B b [bi:] C c [si:]
D d [di:] E e [i:] F f [ef]
G g [d3i:] H h [eit∫] I i [ai]
J j [d3ei] K k [kei] L l [el]
M m [em] N n [en] O o [əu]
P p [pi:] Q q [kju:] R r [ɑ:]
S s [es] T t [ti:] U u [ju:]
V v [vi:] W w [′d∧blju:] X x [eks]
Y y [wai] Z z [zi:][zed]
如：Please pay attention to your spelling.You have dropped an ”m” here.
因为字母答案选m的发音是以元音[e]开头的，所以用an这里的an的意义与one相当，表示“一个”.
又如：
Five years ago her brother was a university student of physics.
因为university是以辅音[j]开头的要用a，physics是学科名词，前面不用冠词.
