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