Two recent related questions prompted this post: one on whether “funner” is a word, and one on “stupider” vs. “more stupid.” (Thanks, @kellidubya and @joshwood)

These forms are called comparatives: adjectives that, as the name implies, indicate a comparison between two entities, times, states of being, etc.

Maria is the younger of the twins.
Scott is happier now that he learned to rumba.
Is the sky over Kansas really bluer?

In English, comparatives are formed in one of two ways:
1. Adding “-er” to the adjective: older, faster, greener, smarter
2. Putting “more” in front of the adjective: more ancient, more rapid, more verdant, more intelligent

There’s not a hard and fast rule — this is English we’re talking about — governing when to use “-er” and when to use “more.” But typically, shorter adjectives take “-er” and longer ones take “more” — “funner” being one of the exceptions to that guideline. “More” is acceptable with any adjective, but it usually sounds odd when used with one that can take “-er.” Some comparatives sound fine either way, like “stupider” and “more stupid.”

Don’t use both “-er” and “more,” though — “more yummier” is fine if a 5-year-old says it, but adults should know better.

Adverbs can have comparatives, too, but most of the time they’re formed with “more” (i.e., people read more quickly than they talk).

On to superlatives: When you’re talking about more than two things, you need superlatives, which are the “-est” and “most” forms. The same guideline for comparatives applies: shorter adjectives form a superlative with “-est” and longer ones with “most.”

Ophelia is the oldest of the triplets.
That is the most frustrating package to open.
The sky today is the bluest I’ve ever seen it.

Do you feel smarter?