6XXused to mean 'each' when stating prices, rates, or speeds:
I get paid once a month.
The eggs cost $2 a dozen.
7XXused before singular nouns to mean all things of a particular type:
A square has four sides (=all squares have four sides).
A child needs love and affection.
8XXused once before two nouns that are mentioned together very often:
I'll fetch you a cup and saucer.
Does everyone have a knife and fork?
9XXused before the -ing forms of verbs when they are used as nouns referring to an action, event, or sound:
There was a beating of wings overhead.
Bernice became aware of a humming that seemed to come from all around her.
10XXused before nouns that are usuallyuncountable when other information about the quality, feeling etc is added by an adjective, phrase, orclause:
Candidates must have a good knowledge of chemistry.
11XXused before the name of a substance, food etc to refer to a particular type of it:
Use a good cheese to make the sauce.
plants that grow well in moist soil
12used before the name of a drink to refer to a cup or glass of that drink:
Can I get you a coffee?
Renwick went to the bar and ordered a beer.
13XXused before the name of a famous artist to refer to a painting by that artist:
an early Rembrandt
14XXused before a name to mean someone or something that has the same qualities as that person or thing:
She was hailed as a new Marilyn Monroe.
15XXused before someone's name when you do not know who they are:
There is a Mr Tom Wilkins on the phone.
16XXused before the names of days, months, seasons, and events in the year to refer to a particular one:
We arrived in England on a cold wet Sunday in 1963.
I can't remember a Christmas like it.
WORD CHOICE: WORD CHOICE: a, an Before a word beginning with a vowel sound, use an• an elephant • an umbrella • an obvious mistake!! Use anbefore an 'h' that is not pronounced• an hour later• an honest explanation!! Use abefore a 'u' that is pronounced like 'you'• a university • a unique opportunity!! Use anbefore an abbreviation that is pronounced with a vowel sound at the start• an SOS call• an MP3 file
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.