Gradable and Non-gradable Adjectives

Adjectives can be either gradable or non-gradable
Gradable
Gradable adjectives are adjectives like ‘cold’ ‘hot’ and ‘frightened’. You can be very cold or a bit cold. Gradable adjectives show that something can have different degrees.
Non-gradable
Non-gradable adjectives are adjectives like ‘married’ or ‘wooden’. You can’t be very married or a bit married. Non-gradable adjectives do not have different degrees. Adjectives like ‘terrifying’, ‘freezing’ ‘amazing’ are also non-gradable adjectives. They already contain the idea of ‘very’ in their definitions – ‘freezing’ means ‘very cold’ etc.
Using adverbs of degree
When we use adverbs of degree to modify adjectives we usually have to use different adverbs for gradable and non-gradable adjectives.

• NOT I’m completely hot.
• NOT It was very fantastic!

With gradable adjectives

• It’s a bit cold in here. Shall I turn the fire on?
• He’s very interested in history. Why don’t you buy him a history book?
• This exercise is really difficult. I don’t know any of the answers.
• I’m extremely tired. I’m going to bed. The adverbs ‘a bit’, ‘very’, ‘really’, ‘extremely’ and quite can all be used with gradable adjectives.

With non-gradable adjectives

• It’s absolutely freezing in here. Shall I turn the fire on?
• He’s completely fascinated by history. Why don’t you buy him a history book?
• This exercise is absolutely impossible.
• That film is really terrifying. Don’t go and see it on your own.

The adverbs ‘absolutely’ and ‘completely’ can be used with non-gradable adjectives. Notice that ‘really’ can be used with both gradable AND non-gradable adjectives but it is normally used in less formal speaking and writing.
© British Council

http://learnenglish.britishcouncil.org/en/quick-grammar/adjectives-gradable-non-gradable