Adjectives: More, most or -er, -est?

When we want to make a comparative or a superlative  of an adjective, do we add  -er and -est  or precede the adjective  with more and the most?

Rule:
If  the "positive" (i.e. the ordinary) form of the adjective has one syllable we add -er and -est (oh, and don't forget the the before the superlative)

Big-bigger-the biggest
Fast-faster-the fastest
Small-smaller-the smallest
Green-greener-the greenest
Sweet-sweeter-the sweetest

If the positive form has more than two syllables, we use more and the most.

Interesting-more interesting-the most interesting
Fantastic-more fantastic-the most fantastic
Ingenious-more ingenious-the most ingenious
Natural-more natural-the most natural

Easy, no?

No, because, as you may have noticed, I  slyly left out what to do when the positive form of the adjective  has two syllables

Here the rule becomes complicated. Usually, we should use more and most, but if the adjective ends in y (and a surprisingly large number of two-syllable adjectives do), then the y becomes an i and we add -er and -est

Easy-easier-the easiest
Lazy-lazier-the laziest
Crazy-crazier-the craziest
Hazy-hazier-the haziest
Foggy-foggier-the foggiest

If it ends in an unstressed syllable we also add -er and -est

Narrow-narrower-narrowest
Quiet-quieter-quietest
Little-littler-the littlest
Subtle-subtler-the subtlest
Clever-clever-the cleverest

Unless, of course,  the unstressed syllable is -ful

Hopeful-more hopeful-the most hopeful
Faithful-more faithful-the most faithful

- This is getting complicated - can't I just make it up as I go along?

Sort of - at least with the two-syllable ones, especially when speaking. It is something that English has never quite resolved.

Here's Shakespeare (As You Like It)

Now, my co-mates and brothers in exile,
Hath not old custom made this life more sweet
Than that of painted pomp? 

Surely he meant sweeter?