8a. The forms of modifiers change as they are used in comparison.
Regular Comparison
A modifier of one syllable regularly forms its comparative and superlative by adding -er and -est.
Some modifiers of two syllables form their comparative and superlative degrees by adding -er and -est; other modifiers of two syllables form their comparative and superlative degrees by means of more and most.
Modifiers having more than two syllables form their comparative and superlative degrees by means of more and most.
Irregular Comparison
Comparative and superlative degrees are formed using completely different words
8b. Use the comparative degree when comparing two things; use the superlative degree when comparing more than two.
8c. Do not omit the words other or else when comparing one thing with a group of which it is a part.
8d. Avoid double comparisons.
Example: do not write “more friendlier”.
8e. Be sure your comparisons are clear.
Dangling Modifiers
8f. A modifying phrase or clause that does not clearly and sensibly modify a word in the sentence is a dangling modifier.
8g. Modifying phrases and clauses should be placed as near as possible to the words they modify.
Aren't quite sure? Click here for the full text (with explanations and examples).