How to Form the States of Adjectives
You ask, therefore, how to tell when to use the ‑er and ‑est endings and when to use the helping words more and most? There’s no hard and fast rule, but some rules of thumb will help:
Syllable Rule for States of Adjectives
One-syllable adjectives (In almost all cases, use ‑er and ‑est.)
big, bigger, biggest
small, smaller, smallest
thin, thinner, thinnest
Note: In some expressions, however, even one-syllable adjectives use more to form the comparative: more sweet than sour.
Two-syllable adjectives (In many cases, use ‑er and ‑est.)
happy, happier, happiest
narrow, narrower, narrowest
silly, sillier, silliest
Two-syllable adjectives (Some always require more and most.)
bizarre, more bizarre, most bizarre
steadfast, more steadfast, most steadfast
Two-syllable adjectives (Some can use either the suffixes or the more-most technique.)
clever, cleverer, cleverest or
clever, more clever, most clever
cruel, crueler, cruelest or
cruel, more cruel, most cruel
Three-syllable adjectives (Use more and most.)
difficult, more difficult, most difficult
memorable, more memorable, most memorable
Participle Rule for States of Adjectives
Present participles (‑ing verbs) and past participles (‑ed verbs) can act as adjectives.
thrilling movie, tired golfer
When you show these participial adjectives in comparative or superlative states, always use more and most.
more thrilling movie
more tired golfer
Sounds-Weird Rule for States of Adjectives
Sometimes the ‑er and ‑est endings just sound strange.
For example, the correct comparative and superlative forms of common are commoner and commonest. But these words sound strange, the first like some cockney character pushing a barrow in the East End of London, the second like some sort of dangerous subversive.
Thus: common, more common, and most common would be regarded as correct by most people (simply because they’re used to hearing these forms). In formal settings, however, you should choose the correct forms, commoner and commonest.
Look-It-Up Rule for States of Adjectives
You can always find the answer in the dictionary.
Look up common, and immediately following the word you’ll find the endings ‑er and ‑est. These entries show how to form the comparative and superlative forms of the adjective.
Now look up the word different. Notice that following the entry you do not find the endings ‑er and ‑est. The absence of these endings means that you use more for the comparative and most for the superlative.