Question

The superlative form of an adjective should only
be used when comparing what

Answer 1

The superlative form of an adjective should only be used when comparing more than two things.

Answer 2

The superlative form of an adjective should only be used when comparing three or more nouns. The superlative form of an adjective is usually denoted by "most" before the adjective or the suffix "-est". A comparative form of an adjective is used to compare 2 nouns only. Comparative adjectives have the suffix "-er" or the word "more" before the adjective.