If the adjective ends in y and is two syllables long you can drop the y and use er and est. Otherwise you use more and most in front of the adjective.

So, happy, happer, happest? That's some rule.

The older forms of comparison, i.e., -er and -est, seem more restricted in their use. Many people do seem to use securest, so I wouldn't call it incorrect exactly. How about a reverse question, can one use the more and most forms with shorter adjectives? Quiet?

The most quiet place in the world.

The quietest place in the world.

I'll probably be beaten unconscious with a bloody copy of Shooting Leaf-eating Pandas, but it's worth the trouble.

That was more fun than a barrel of grammatical monkeys.

Ceci n'est pas un seing.