a an the Acronym Initialism Abbreviation

用a或an取缺於一個字(word)開始的讀音, 不是字母(ABC...Z) an用於母音(vowel)開始的單字之前, 其它用a

an apple, 

a boy

a house, an hour

a historic, an historic (兩者皆可)

如果個字屬於Abbreviation(縮寫), 就要分開它是Acronym還是Initialism.

Acronym是整個字發音, 如NASA, 讀Na-Sa

Acronym不用 (the) 

an AIDS

a laser

Initialism是每個字母發音, 如MBA, 讀Em, Bee, A

Initialism可用 (the)

a USB

an MBA

an A (A)
a B (bee)
a C (cee)
a D (dee)
an E (ee)
an F (eff)
a G (gee)
an H (aitch)
an I (eye)
a J (jay)
a K (kay)
an L (el)
an M (em)
an N (en)
an O (O)
a P (pee)
a Q (cue)
an R (ar)
an S (ess)
a T (tee)
a U (U)
a V (vee)
a W (double you)
an X (ex)
a Y (wye)
a Z (zed)