The

The is used:

when there is only one of something in a particular area and everyone knows about it: 

the government, the police, the bridge, the river, the hospital

when there is only one in the entire world:

the internet, the environment, the ozone layer, the atmosphere

with cardinal numbers: 

the first, the second, the third

with superlatives: 

the worst, the shortest, the lowest, the most beautiful, the least impressive

places where the name refers to a group of islands or states: 

the USA, the UK, the Maldives, the Middle East, the United Arab Emirates

before nouns which describe general things: 

Exercise is good for the body.

The motorbike is the most common form of transport in Asia.

The role of the teacher has changed in recent years.

before abstract nouns used to describe a situation, process, quality or a change: 

Over the years, the development of the town accelerated.

The frequency of violent crime decreased over the period.

The improvement in living standards...

We don't use the:

to talk generally. We drop the word ‘the’ and use the plural form: 

Dogs don’t like cats.

People with dyslexia have reading problems.

Japanese cars are very reliable.

German products are very high quality.

with a single place or country: 

Ireland, China, Vietnam, Europe, South America