Adjectives describe qualities (characteristics) of nouns. Some qualities can vary in intensity or "grade", for example:
rather hot, hot, very hot; hot, hotter, the hottest
The adjective hot is gradable.
Other qualities cannot vary in intensity or grade because they are:
extremes (for example: freezing)
absolutes (for example: dead)
classifying (for example: nuclear)
The adjectives freezing, dead and nuclear are non-gradable.
A gradable adjective can be used with "grading adverbs" that vary the adjective's grade or intensity. Look at these examples:
a little, dreadfully, extremely, fairly, hugely, immensely, intensely, rather, reasonably, slightly, unusually, very
Gradable adjectives" are also called "qualitative adjectives". "Grading adverbs" are also called "submodifiers".
A gradable adjective can also have comparative and superlative forms:
big, bigger, the biggest
hot, hotter, the hottest
important, more important, the most important
Look at these example sentences:
My teacher was very happy with my homework.
That website is reasonably popular. But this one is more popular.
He said that France was a little cold and Denmark was rather cold. But Sweden was the coldest.
A non-gradable adjective cannot be used with grading adverbs:
It was rather freezing outside.
The dog was very dead.
He is investing in slightly nuclear energy.
Non-gradable adjectives do not normally have comparative and superlative forms:
freezing, more freezing, the most freezing
dead, deader, the deadest
nuclear, more nuclear, the most nuclear
Don't try to learn lists of gradable and non-gradable adjectives! It's better to understand what makes an adjective gradable or non-gradable. This is a matter of logic and common sense. Most native-speakers have never heard of gradable and non-gradable adjectives. They just "feel" that it doesn't make sense to say "fairly excellent" or "very unique". You probably have the same idea in your language.
Often, non-gradable adjectives are used alone:
It was freezing outside.
The dog was dead.
He is investing in nuclear energy.
However, a non-gradable adjective can be used with "non-grading adverbs" (which usually just give the adjective extra impact), for example:
Here are some example sentences containing non-grading adverbs with non-gradable adjectives:
Her exam results were absolutely awful. She will have to take the exam again.
Is there anything like it in the world? It must be virtually unique.
Gradable / Non-gradable Adjectives
Some adjectives may have more than one meaning or sense. It's possible for the same adjective to be gradable with one sense and non-gradable with another sense. For example:
Adverbs used with gradable and non-gradable adjectives
The adverbs really (very much) and fairly and pretty (both meaning "to a significant degree, but less than very") can often be used with gradable and non-gradable adjectives:
"Quite" with gradable and non-gradable adjectives
The sense of the adverb "quite" changes according to the gradability of the adjective that we use it with: