17 – System or service
We use the when talking or speaking about a system or service.
e.g. the radio, the police, the school, the weather forecast, etc.
Turn off the radio.
Call the police!
The stadium is adjacent to the school.
What was the weather forecast?
18 – Adjectives to discuss groups
We use the when using adjectives to discuss groups.
e.g. the rich, the poor, the employed, the unemployed, the homeless, etc.
She is a friend of the poor.
The rich sometimes despise the poor.
I hear that the number of the unemployed has been increasing recently.
She spoke for the homeless.
19 – All things
When we talk about something that is a feature of all types of that thing we use the.
The largest animal on Earth is the blue whale.