an
This is used when the noun that we wish to refer to is unknown to our listener/reader or is not part of the common ground that we share. It is most often used to introduce new information.
Tell me a story.
Have you ever seen a tornado?
The
By using the, we are signalling to our listener that s/he is very likely to know what we are referring to and that the context of our conversation should help them to identify this. We can use the, therefore, to
refer backwards to something that we have already mentioned
refer forwards to something that we can take for granted will happen
refer to our common ground or shared knowledge
Here are some examples to illustrate each of those contexts:
I was out the other day and I found a ten-pound note on the street. I couldn't decide whether to keep the money or hand it in. (I have already talked about this money in the previous sentence.)
We'll need to take an axe to cut the trees . (i.e. those trees that we find in the place that we are going to.)
Have you put the cat out? (i.e. our cat)
The is also used with certain fixed expressions where there is often common knowledge, for example places of entertainment, oceans and seas, hotels etc. the Alps, the cinema, the Pacific Ocean, the Hyatt and even for some more generic tersms such as 'the High Street', 'the open seas'.
Note also that the is sometimes (but not always) used with some countries' names, such as:
She lives in England, which is part of the UK.
He visited the Czech Republic.
The DRC (Democratic Republic of Congo) has experienced strife for many years.
I think the Philippines is a beautiful country.
Jack loves the Netherland and works in the Hague.
In addition, when referring to some named or unnamed organizations, for example:
He was arrested by the FBI.
She works for the BBC.
The U.N. has its headquarters in New York.
He left home and joined the army.
No article
If we want to refer to something general and the nouns that we are using are either plural or uncountable, we leave out articles.
I really like funfairs. ('funfairs' in general; I have no specific funfair in mind.)
It's brought us nothing but trouble. (uncountable noun)
Some of the other times when an article is not needed are:
with proper nouns like people's names, countries, towns, cities, single mountains, streets, lakes, and countries (but see note above about certain countries and places).
meals - when are we having lunch, I have cereal for breakfast.
certain time expressions - next year, last month, this week, on Friday, at five o'clock.
in an institution - he's in prison, I'm at school next week, do you go to university.