We use the to refer to a specific something. So we use the only when everyone knows what you refer to. When there is only one thing you could mean.
The man was following me.
I grabbed the chair.
I walked past the school.
He grabbed the laptop.
We usually use the when something is referred to for the second time.
For example:
- An apple fell off the tree. He grabbed the apple and ate it.
- A dog was barking. I told the dog to shut up.
- Oh my gosh! A child got hit by a car! The car was driving too fast.