- adj + -er (than) when you want to compare two people or two things and the adjective has got one syllable or  two syllables and ends in -er, -y, -ow, -le:

The man on the right is stronger than the man on the left. The man on the right is happier than the man on the left because he is very strong.

  - more + adj. (than) if the adjective has got two syllables and ends in -ful, -re, is a participle used as an adjective, or has three or more syllables:

Paul is very intelligent. John is intelligent.  Paul is more intelligent than John.
Robert is very careful. Jonathan is careful. Robert is more careful than Jonathan.

  - (the) adj. + -est (of / in) when you want to compare more than two people or things and the adjective has got one or  two syllables and ends in -er, -y, -ow, -le:

Paul is the tallest boy in the class.  Jonathan is the cleverest student in the class.

  -  (the) most + adjective (of / in) if the adjective has got two syllables (sometimes), three or more syllables:

Paul is the most intelligent boy in the class.

SPELLING:
- final -y + -er = -ier:  heavy - heavier
- final -e + -er = -er:  brave - braver
- one syllable adjectives ending in a single vowel + single consonant + -er  = double consonant: big - bigger


Comparison in relation to the same degree is expressed by as...as (when the verb is in the negative form so...as can also be used):

Paul is 1.80m tall. Robert is 1.80m tall.    Paul is as tall as Robert.

Comparison in relation to a lower degree  is expressed by less or least (depending on the number of people or things you want to compare). less is used when you compare two people or things and least is used if you compare more than two people or things:

Unit 5 is less difficult than unit 4.   In fact, unit 5 is the least difficult of all.