Comparatives vs. Superlatives

Comparatives and superlatives fall under the category of "adjectives". Their function is to show the comparison between one thing and another. Here are simple explanations on how to differentiate the usage of comparative and superlative, taken from ESLBase. 

One syllable adjectives
Comparative: add er (cheaper)
Superlative: add est (the cheapest)

One syllable adjectives ending in 'e'
Comparative: add r (nicer)
Superlative: add st (the nicest)

One syllable adjectives ending in consonant - vowel - consonant
Comparative: add consonant + er (hotter)
Superlative: add consonant + est (the hottest)

Two syllable adjectives ending in 'y'
Comparative: replace 'y' with ier (happier)
Superlative: replace 'y' with iest (the happiest)

Two or more syllable adjectives
Comparative: add more / less (more / less beautiful)
Superlative: add the most / the least (the most / least beautiful)

Irregular adjectives
good - better - the best
bad - worse - the worst
far - further - the furthest

However, students can easily confuse the usage of both forms. Here are common errors people make.

Bigger vs. Biggest
Don't say: Which of these two guys has the biggest feet?
Say instead: Which of these two guys has the bigger feet?
Why?: You need to use the comparative form when comparing two things.

Less vs. Least
Don't say: Of all the movies, that one seems less deserving of the award.
Say instead: Of all the movies, that one seems least deserving of the award.
Why?: "Of all the movies" indicates that there are more than two movies, thus the superlative form "least" is used.

Better vs. Best
Don't say: The best of the two golfers actually lost the game.
Say instead: The better of the two golfers actually lost the game.
Why?: "Better" is the comparative form to compare two things.

More vs. Most
Don't say: I think he's more smarter when he's at the office.
Say instead: I think he's smarter when he's at the office.
Why?: "More" is the comparative and "most" is the superlative. You must use "more" and "most" with modifiers that don't have "-er" and "-est" forms. You say "more intelligent" or "most intelligent", not "intelligenter" or "intelligentest". You should never, ever add "-er" or "-est" endings to a word that you're also modifying with "more" or "most". It is never "more smarter" or "most smartest", because then you have two words doing the same job. More examples;

She is the prettiest of all their children.

She is the most beautiful of all their children. 

This is the most vicious review I've ever read.

This is the cruelest review I've ever read.

His story was shorter than Tom's.

His story was more interesting than Tom's.

Try this little test to know whether you have understood this well :)

Pick the correct choice.

1. Both professors had published a book that week, so they spent the entire evening arguing over which book was (bigger, biggest).
2. Which of the Three Stooges has the (bigger, biggest) following among males?
3. Of all their 6 cats, I think Fluffy is (less, the least) likely to do something embarrassing in front of Aunt Nini.
4. Ronaldo couldn't decide between his two evil schemes; the first was (less, the least) difficult, but the second was (more, the most) fiendish.
5. Rory will be (happier, more happier) when the kids are out of the house and she can rest all day.
6. It was the (tastiest, most tastiest) anchovy-and-onion pizza they had ever eaten. 

Answers are in the comment section!