a an分別.

Use A before a word beginning with a consonant.
e.g. a man, a car , a boy, a horse, a house

Use AN before a word beginning with a vowel. (a, e, i, o, u)
e.g. an island, an umbrella, an uncle, an egg, an apple


Pay attention to the following exceptions. It is advisable to memorize them.

Use A before a word beginning with a vowel with consonant sound.

a European, a Eurasian, a euphemism, a eulogy, 
a university (pronounced as “you”), a union, a unit, a U-turn, a uniform, 
a unique explanation
a one-way street, a one-man show, a one-piece swimsuit, a one-shot deal,
a one-stop shop

Use AN before a word beginning with a consonant with vowel sound, or individual letters spoken with a vowel sound.

an hour (silent h, pronounced as “our”), an honour
an honest man (silent h, pronounced as “on-nest”)
an L-plate, an MP, an SOS, an “x”, an X-ray
 
Hope that can help.

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其實a 同 an要點用.
An is used when the noun or a word is begun with an vowel - A E I O U or when a noun is pronounced as en em on etc.

An apple a day keeps doctors away. apple is begun with A
I takes an hour to get there. Hour is pronoumced as our
An MP3 is a device, which we used to listen to music. MP3 is pronounced as em P three

A red apple is what Mary wants. Red is not begun with R SO A is used.

A mobile phone is what Tom needs. Mobile is pronounced as mo so A is used.

I hope you will understand the basic rule for using a & an.