The choice of "a" or "an" depends on the vowel sounds, not on the vowel letters "a, e, i, o, u." Pay attention to the following words and phrases:
When the letter "u" is pronouced as the sound of letter "u", "a" is used.
an umbrella
a university
a UFO
a utensil
an utterance
an uncle
an ugly dog
a useful box
a uniform
a unit
an ulcer
If "h" is not pronounced at the beginning of a word, "an" is used.
an hour
a house
a half
an honor
a hotel
a hand-gun
a horror movie
Starting with a consonant letter, but "an" is used.
an SUV (starting with a vowel sound)
an F grade (starting with a vowel sound)
Starting with a vowel letter, but "a" is used.
a European (starting with "u" sound)
a one-bedroom apartment (not starting with a vowel sound)
a one-time password
The Uses of the Article "THE"
The word "the" actually means this or that, these or those. If you talk about something specific, use "the"; if in general, without using "the".
e.g. The women are sisters. / Women also have rights to vote.
For certain nouns, you must always use the article "the."
There is only one, unique
e.g. the earth, the moon, the universe
Talking about the direction
e.g. in the east, in the north, in the south
Before the name of a season
e.g. in the spring, in the summer
The superlative degree
e.g. Monday is the busiest day of a week.
Before an adjective to mean a group of people
e.g. the rich, the elderly, the aged, the Smith
Before an ordinal number
e.g. the first, the second, the third
Before rivers, oceans, seas, gulfs, ranges of mountains, deserts, forests
e.g. the Nile, the Pacific, the Sahara, the Persian Gulf
Before the full name of a country
e.g. the People's Republic of China, the United States
Do not use "the" before:
Names of places: Los Angeles, Monterey Park, Pasadena, Beijing
Names of holidays: Mother's Day, Thanksgiving, Christmas, Valentine's Day. Easter
Names of streets: Valley Blvd, Fairview Avenue, Fifth Street
Names of continents: Asia, Africa, Europe
Names of countries (single-word names): China, Brazil, Italy
Names of mountains: Mount Everest, Mount Fuji
Names of languages: Chinese, English, French
Common activities: go to school, go to hospital, go to church, go to prision, at school, at work, at home, in prison, in bed, on foot