Gradable and Non-gradable adjectives

In case my explanation wasn't really enough, I've found this one that looks simple and clear.

Let’s look at gradable and non-gradable adjectives in turn briefly, then at the issue of adverbs. For those students you mention who need rules, here come the rules…

Gradable adjectives Many adjectives describe qualities that can be measured in degrees, such as size, beauty, age, etc. These adjectives are often called gradable adjectives, because they can be used in comparative or superlative forms, or with grading adverbs such as very or extremely, to show that a person or thing has more or less of a particular quality. The following are some examples of common gradable adjectives used with grading adverbs:

Don’t talk to Jenny, she’s very angry.

Sorry, I can’t stop, I’m extremely busy.

I’m worried about Tom, he’s dreadfully unhappy.

This is a very important matter.

That slice of cake is rather big.

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Non-gradable adjectives
Some adjectives describe qualities that are completely present or completely absent. They do not occur in comparative and superlative forms, and cannot be used with adverbs such as very or extremely, because we don’t usually imagine degrees of more or less of the quality being described. They are referred to as non-gradable adjectives. Non-gradable adjectives do sometimes occur with non-grading adverbs such as completely which emphasize the extent of the quality, e.g.:

The questions were completely impossible.

The food was absolutely superb.

Adjectives which identify something as belonging to a particular type, sometimes referred to as classifying adjectives, are also non-gradable, cf:

a chemical reaction. = an extremely chemical reaction

an indoor pool = a very indoor pool

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Gradable and non-gradable
Note that, sometimes, confusion arises because some adjectives have both gradable and non-gradable senses with similar meanings e.g.:

She sounded slightly foreign.

He’s advising on foreign policy.

 In the first example, foreign is gradable because it refers to a measurable quality, whereas in the second, foreign is non-gradable because it refers to a particular type (of policy).

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Which adverb to use?
As a rule of thumb, the common grading adverbs extremely, very, rather, slightly and a bit, which emphasize degrees of a particular quality, don’t usually occur with non-gradable adjectives, since non-gradable adjectives identify a particular type or an absolute quality. Compare the following examples with non-gradable and gradable adjectives:

non-gradable	gradable
This food is extremely superb.	This food is extremely tasty.
I found a very Roman coin.	I found a very shiny coin.

On the other hand, common non-grading adverbs such as completely, totally and absolutely, which emphasize absolute qualities, do not usually occur with gradable adjectives, since gradable adjectives describe qualities which can be measured in degrees. Compare the following examples with gradable and non-gradable adjectives:

gradable	non-gradable
This book is absolutely excellent.	This book is absolutely interesting.
The exercise was completely impossible.	The exercise was completely difficult.

One further important thing to remember is that the commonly used adverb really, and the adverbs fairly and pretty (an informal variant of fairly) are regularly used with both gradable and non gradable adjectives, e.g.:

It’s a really difficult/impossible exercise.

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