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