Absolute adjectives

Some adjectives express ideas that cannot be graded. For example, a person can’t be more or less dead. In the same way, a sphere can’t be more or less round. In grammars these adjectives are called non-gradable or absolute adjectives.

Non-gradable adjectives do not have comparative or superlative forms. There are very few non-gradable adjectives, so you can learn them by heart if you really want. Here is a list of common non-gradable adjectives in English. Note that this is not a comprehensive list.

Absolute, impossible, principal, adequate, inevitable, sufficient, complete, main, unanimous, unavoidable, entire, minor, fatal, unique, final, universal, ideal, whole, preferable, dead etc.                      
Exceptions

Although the adjectives given above are not normally used in comparative and superlative forms, you might still hear expressions like more complete or most perfect. Though incorrect, these expressions are quite common in speech and they have become sort of acceptable, too. However, if you are a careful user of the language, you must avoid them especially in writing.

Also be careful, not to use more along with a comparative adjective ending in –er and most with a superlative adjective ending in -est. Do not write more taller or most smartest. These are examples of double comparatives and superlatives. They are always wrong and must be avoided.

• She is prettier than her sister. (NOT She is more prettier than her sister.)

Some compound adjectives have two possible comparatives and superlatives.

Positive: good-looking

Comparative: better-looking or more good-looking

Superlative: best-looking or most good-looking

Positive: well-known

Comparative: better-known or more well-known

Superlative: best-known or most well-known

In this case, you can decide which form you want to use, but don’t write most best-known or more better-looking.