To know how to form comparatives and superlatives in English, you must be able to count how many syllables the adjective or adverb you want to use contains.

*Remember that adjectives generally describe nouns, whereas adverbs may describe nouns, verbs, or other adverbs. Therefore, we use adjectives to compare people, places, and things; we use adverbs to compare actions.

“Sweet” has one syllable.
“Easy” has two syllables. “Ee – zee”
Interesting and beautiful have three or more syllables.

Clap your hards as you pronounce each part of the word, and you will be able to count syllables without any problems.

We use comparatives to compare two different things.

Comparatives and superlatives are paired with articles “the” and “a/an” in predictable ways.

We use superlatives to express that, out of a group of three or more, one is the most or least in absolute terms. When you learned how to use the article “the,” you understood that we use “the” with superlatives to say that something is the most or the least or the ugliest in absolute terms among all of the given choices. 

Likewise, we often use the article “a” to make a comparison.
Example: She is a better tennis partner than he is.
*In this case “a” means “an example of something.”

When we want to compare a quality that has one syllable. We simply add the ending “-er.”
One store’s gelato is sweeter than the other store’s gelato.
If we want to use the superlative to make an absolute statement, we add –est.
This store’s gelato is the sweetest in Parma.

There are some annoying spelling rules for comparatives and superlatives.
If the quality you want to compare has one syllable, but already ends in -e, just add -r.
Example:
This math problem is simple. This math problem is simpler than that one.
If you want to use a superlative to speak about it in absolute terms, just add the -st.
Why can’t your boyfriend answer even the simplest question?

If the quality ends with one vowel (a,e,i,o,u) and one consonant, you must double the last consonant.
My cat is fat. His cat is thinner than mine.
My cat is the fattest cat that I have ever seen in my life.

For two syllable adjectives that end in -y, delete the -y and add the ending -ier to make a comparison and -iest to create the superlative.
Example:
The homework was easy. Your group’s homework was easier than our group’s homework. It is not fair that your group’s homework is always the easiest.

When we want to compare a quality that has two syllables, we sometimes add the ending “-er” and sometimes use the qualifying words “more” or “less.” There is a list of these that you need to memorize. It mostly is a matter of pronunciation and what sounds better.
Cleverer would not be easy to pronounce, so we say that someone is more or less clever if we are comparing two people. We say they are the most or least clever if we are speaking about them in absolute terms in regards to a group.

The same is true for all qualities that have three or more syllables.
We never say that something is interestinger than something else. It does not sound good.
We say, “He is more interesting to me than my current boyfriend.”
Or, “He is the most interesting guy that I have met in a long time.”

There is also a list of exceptions that are irregular:
bad – worse – worst
good – better – best
little – less – least
much – more – most
far – farther – farthest