Indefinite articles A and AN

are used before singular countable nouns

is used before a singular noun beginning with a consonant
a cat
a bike

is used before a singular noun beginning with a vowel
an elephant
an umbrella

is used before singular and plural nouns when the noun is specific or particular
I can see a scooter.
The scooter is blue.

is used with nouns that are unique
the moon, the sky,
the sun, the Pope, etc.

is used before the names of rivers, seas/oceans, mountain ranges, deserts and groups of islands
the Amazon, the Pacific,
the Alps, the Sahara,
the Canaries, etc.

is used before countries whose names contain words like kingdom, states or republic
the United Kingdom,
the United States,
etc.

is used before the names of musical instruments
the flute, the violin,
etc.

is used before nationalities
and names of families
the British, the Italians, the Jacksons, the Greens
etc.

is used with superlative adjectives
Mike is the tallest boy in our class.

is used with hotels, pubs, restaurants, cinemas, theatres
the Ritz Hotel, the Odeon, the Palace Theatre,
etc.

is used when the place name contains of-phrase
the Statue of Liberty,
the Tower of London,
etc.

before proper nouns
This is the Steve.
x

with continents, countries, towns, streets or lakes
Asia, Germany, Mardid, Oxford Street, Lake Michigan,
etc.

with names of mountains or islands
Mount Everest, Mount Teide, Sicily, Bermuda, etc.