Use an article.

Example

Use “the” with names for military or public protection agencies.

He served in the army for 20 years.

I haven’t seen the police in this area for a long time.

She was a member of the FBI.

Use “the” with “best,” “most,” and “-est” adjectives

That was the best movie. He is the most talented director. That’s why he has the biggest name in the business.

Use “a” when  comparing things or people

She is a better person than I am.

Use “the” with the names of families.

The Johnsons live next door.

Use “the” for abstract groups.

It can be hard to work with the poor, but worthwhile.

Use “the” with classes of musical instruments and classes of animals.

The violin is a special instrument. 
The hyena is a violent predator.

Use “a,” “an” or “the” if the instrument or animal is a regular noun.

I have a piano in the living room.
I saw a hyena on TV.

Use “the” for names of rivers, oceans, seas, deserts, regions.

I have never seen the Mississippi river.

The Pacific Ocean is very beautiful.

The east coast is different from the west coast.

Use “the” with names of bridges, buildings, zoos, gardens, museums, and companies.

We stayed at the Westin Hotel.

We drove across the Golden Gate Bridge.

You should visit the San Diego Zoo.

Use “a” for one of a chain of hotels. Use “the” to mean a specific hotel, even if it’s a chain.

We stayed at a Hilton last night.

They stayed at the Westin Hotel in downtown.