Here is a list of common two-syllable adjectives whose comparatives and superlatives are usually formed like this:
busy
dirty
clever
easy
funny
happy
heavy
lovely
lucky
pretty
quiet
silly
simple
steady
tiny
R24 Some other two-syllable adjectives usually have comparatives and superlatives formed with more and most.
careful
Here is a list of common adjectives whose comparative and superlative forms are usually formed with more and most:
careful
common
famous
handsome
likely
mature
obscure
pleasant
polite
sudden
R25 Many two-syllable adjectives can have comparatives and superlatives with either the endings er and est, or more and most. In many cases, the er and est forms are more commonly found directly before the noun (in attributive position), and the more and most forms more commonly follow a linking verb such as be or become (in predicative position). For more information about attributive and predicative adjectives, see paragraphs 2.42 to 2.52.
…major hurricanes such as Katrina, the costliest disaster in U.S. history.
Energy is becoming more costly and supplies are drying up.
Less space seemed to make for a friendlier neighborhood feeling.
We are encouraging employers to be more friendly to the local environment.
Here is a list of common adjectives that can have either type of comparative and superlative:
angry
costly
cruel
friendly
gentle
narrow
remote
risky
shallow
stupid
subtle
R26 Adjectives that have three or more syllables usually have comparatives and superlatives with more and most.
dangerous
However, some three-syllable adjectives are formed by adding un to the beginning of other adjectives. For example, unhappy is related to happy and unlucky to lucky. These three-syllable adjectives have comparatives and superlatives formed either by adding er and est or by using more and most.
He felt crosser and unhappier than ever.
R27 When you add er or est to an adjective, you sometimes need to make another change to the end of the adjective as well.
If a one-syllable adjective ends in a single vowel letter followed by a single consonant letter, you double the consonant letter when adding er or est.
big
However, you do not do this with two-syllable adjectives.
clever
If an adjective ends in e, you remove the e when adding er or est.
wide
Note that with adjectives ending in le, the comparative and superlative have two syllables, not three. For example, simpler (from simple /′SImpǝl/) is pronounced /′SImpǝl/.
If an adjective ends in a consonant letter followed by y, you replace the y with i when adding er or est.
dry
Note that with shy, sly, and spry, you add er and est in the ordinary way.
R28 Good and bad have special comparatives and superlatives, which are not formed by adding er and est or by using more and most.
Good has the comparative better and the superlative the best.
There might be better ways of doing it.
This is the best museum we’ve visited yet.
Bad has the comparative worse and the superlative the worst.
Things are worse than they used to be.
The airport there was the worst place in the world.
Note that ill does not have a comparative form and so worse is used instead.
Each day Kunta felt a little worse.
R29 The adjective old has regular comparative and superlative forms but, in addition, it has the forms elder and the eldest. These forms are used only to talk about people, usually relatives.
…the death of his two elder brothers in the First World War.
Bill’s eldest daughter is a doctor.
Note that unlike older, elder never has than after it.
R30 There is no comparative or superlative of little in Standard English, although children sometimes say littler and the littlest. When you want to make a comparison, you use smaller and the smallest.
R31 The comparatives and superlatives of compound adjectives are usually formed by putting more and most in front of the adjective.
self-effacing