Grammar - a 和 an 的用法
语法要点
Article a · before a consonant sound（辅音）
Consonant sound（辅音）是除了 A E I O U 以外的所有音。当名词读出来的第一个音是 consonant sound，前面用 a。
Article an · before a vowel sound（元音）
Vowel sound（元音）是 A E I O U 这五个音。当名词读出来的第一个音是 vowel sound，前面用 an。
Special Case · it's about sound, not spelling
Some words start with a vowel letter but make a consonant sound（辅音）— use a. Some words start with a consonant letter but make a vowel sound（元音）— use an.
a university (starts with /j/ consonant sound)
a uniform (starts with /j/ consonant sound)
an hour (h is silent
starts with vowel sound)
an honest boy (h is silent
starts with vowel sound)
容易搞错的地方
a house
house — h is pronounced, first sound is /h/ consonant（辅音）, use a
I live in a house.
an hour
hour — h is silent, first sound is /aʊ/ vowel（元音）, use an
I waited for an hour.
a university
university starts with /j/ consonant sound（辅音）, use a
She studies at a university.
an umbrella
umbrella starts with /ʌ/ vowel sound（元音）, use an
I need an umbrella today.
a uniform
uniform starts with /j/ consonant sound（辅音）, use a
We wear a uniform to school.
an uncle
uncle starts with /ʌ/ vowel sound（元音）, use an
I visit an uncle every weekend.
a European
European starts with /j/ consonant sound（辅音）, use a
He is a European.
an eagle
eagle starts with /iː/ vowel sound（元音）, use an
I saw an eagle in the sky.
总结
Use a before a consonant sound（辅音）— a cat, a dog, a book, a bus, a tree
"Use an before a vowel sound（元音: A/E/I/O/U）— an apple, an egg, an orange, an umbrella"
It is about the first SOUND, not the first letter
a university / a uniform — spelled with U but start with /j/ consonant sound（辅音）
an hour / an honest — spelled with H but H is silent, start with a vowel sound（元音）