We use a or an...
...with a singular, countable noun mentioned for the first time.
We use the...
...with adjectives to express groups.
We use the...
...with nationalities
We DO NOT use an when...
...the letters "u" or "e" are pronounced with a "y" sound. (a European country)
We use the...
...with things we have mentioned before, or when it´s clear who or what we are referring to from the context.
We use the...
...with things which are unique.
We use the...
...when referring to particular things.
We use an before words beginning...
...with a vowel sound, including silent "h"
We use a or an...
...to talk about jobs.
We DO NOT use a or an...
...with uncountable nouns, or plural countable nouns.