One-syllable adjectives decline, using 'er'. Three-syllable adjectives don't. So what about two-syllable adjectives? Well, it depends. Two-syllable adjectives ending in 'y' take 'er' in the comparative form and 'est' in the superlative. Easier, prettier, friendlier, happier, luckier and so on. Two-syllable adjectives that are verb participles don't. More boring or most pleased, for example. Other two-syllable adjectives can use both forms. For example, you can say "cleverer" or "more clever", "subtler" or "more subtle". We decide on a case-by-case basis, depending on what sounds best.