We use words like very, really and extremely to make adverbs stronger:
She speaks English very well.
They behaved really foolishly.
He put the glass down extremely carefully.
We call these words intensifiers. Other intensifiers are:
We also use enough to say more about an adverb, but enough comes after its adverb:
She didn't win. She didn't play well enough.
We use words like fairly, rather and quite to make adverbs less strong:
She speaks English fairly well.
They behaved rather foolishly.
The children played quite happily.
We call these words mitigators. Mitigators are the opposite of intensifiers.