比較級

more MAIS << >> MENOS less

do que or que = than

e.g.
O tempo de hoje está mais frio (do) que o de ontem. Weather today is colder than weather yesterday.

The superlative

Use definite article in front of superlative:

Hoje é o dia mais frio do ano. Today is the coldest this year.

Some
exceptions:

bom - melhor good - better
mau - pior bad - worse
grande - maior big - bigger
pequeno - menor small - smaller

2002-07-17 sentence corrected

Absolutive superlative

Absolutive superlative can be formed two ways:

analytical (muito before the adjective)

synthetical (-íssimo added to the adjective)

analytical (superlativo absoluto analítico)

Formed by adding adverb before the adjective (usually muito, but other adverbs are used also).

muito caro

NOTICE: This form is used used usually in colloquial speech.

synthetical (superlativo absoluto sintético)

Absolutive superlative is made with íssimo-ending:
caro, muito caro, caríssimo
expensive, very expensive, extremely expensive

NOTICE: The ending of the adjective may change as -íssimo ending is added to it. For example calmo >> calmíssimo, feliz >>felicíssimo.

Irregular absolutive
superlatives:

fácil - facílimo the easiest
difícil - dificílimo the hardest
mau - péssimo the worst
bom - ótimo the best