An intensifier makes an adjective stronger. It comes before an adjective in a sentence.
The story was really interesting.
They are very clever.
She is extremely beautiful.
This is bitterly disgusting.
They are highly qualified teachers.
Note: All the red colour words are intensifiers while the texts in blue colour are adjectives.
Other examples of intensifiers:
Enough is also used to tell more about an adjective, but it comes after its adjective.
If you are a graduate, you are experienced enough to teach kids.
A score of over 350 in cricket is big enough to defend.
You are matured enough to think about the matter.
Intensifiers with strong adjectives
First, you should know what strong adjectives are. These are words like: