Theory:

There are several ways of comparing things in English, so let's start with the simplest.

When adjectives which only have 1 syllable are used to compare something, we need to add the suffix ER followed by THAN:

Joe is BIG.

Joe is BIGGER THAN Dave.

If the adjective has 3 syllables or more, then the word MORE is used rather than the suffix ER, and THAN is still used.

English is DIFFICULT.

English is MORE DIFFICULT THAN Spanish.

There is some confusion about what happens with adjectives with 2 syllables. If it ends in Y, then the suffix ER is used, by first changing the Y to an I. If it doesn't end in Y, then the word MORE is used.

Spanish is EASY.

Spanish is EASIER THAN English.

A car is USEFUL.

A car is MORE USEFUL THAN a bike.

There is another construction which is commonly used to compare things, especially in the FCE exam, and this uses the structure AS ..ADJECTIVE.. AS

To say something is the same:

Joe and Dave are the same size.

Joe is AS BIG AS Dave.

But then we can also say:

Joe is BIGGER THAN Dave.

so, Dave is NOT AS BIG AS Joe.

The structure NOT AS ... AS is used for saying something is less than.

Spanish is NOT AS DIFFICULT AS English.

This is what happens for regular adjectives, but there are some adjectives which are irregular, and these have different rules.

Such irregular adjectives are:

GOOD ...BETTER ...BEST

BAD ...WORSE ...WORST

FAR ...FURTHER ...FURTHEST

Another part of this work is the use of the SUPERLATIVE to express the maximum value of an adjective.

Spanish is EASY.

Spanish is EASIER THAN English.

Spanish is THE EASIEST.

The SUPERLATIVE is constructed by using THE and the suffix EST, for 1 syllable adjectives.

Joe is BIG.

Joe is bigger than Dave.

Joe is THE biggEST.

For adjectives with 3 syllables, THE and MOST are used:

English is difficult.

English is more difficult than Spanish.

English is THE MOST difficult.

For adjectives with 2 syllables, the same rules as before apply, where If the adjective ends in Y, then the suffix ER is used, by first changing the Y to an I. If it doesn't end in Y, then the word MORE is used:

Spanish is easy.

Spanish is easier than English.

Spanish is THE easiEST.

A car is useful.

A car is more useful than a bike.

A car is THE MOST useful.

There are some special cases like GOOD and LIKE.

The adjective GOOD has the comparative BETTER THAN, and the superlative THE BEST, whereas BAD has the comparative WORSE THAN and THE WORST.

It is also used a lot with the adverb WELL:

John is a GOOD dancer.

John dances WELL.

These ideas are also commonly used with the verb LIKE.

PREFER = LIKE MORE = WOULD RATHER + VERB

I PREFER meat to fish.

I LIKE meat MORE THAN fish.

I WOULD RATHER EAT meat THAN fish.

or,

I LIKE meat THE BEST.

Another way of comparing things is to us AS MUCH and AS MANY

I have THREE TIMES AS MANY friends as you.

I lost TWICE AS MUCH weight as you.

The idea of comparing things can also be used to describe things which are very different. An extreme comparison uses the word pair TOO and ENOUGH

So some soup can be HOT, it can be VERY HOT, it can be HOTTER THAN yesterday's soup, and finally it can be TOO HOT, which means it cannot be eaten. This goes together with the word ENOUGH, because if it is TOO HOT, then it isn't COLD ENOUGH.

These ideas are commonly tested in the FCE exam.

SO can sometimes be combined with TOO to express an extreme situation. If the soup was SO HOT that you couldn't eat it, then it was TOO HOT to eat.

Finally,

MORE can be used with an adjective, or with just a noun:

Spanish is MORE DIFFICULT than English.

There are MORE PEOPLE today than yesterday.