Superlatives in Dutch are quite similar to English. They are formed by adding -st to an adjective:

klein → kleinst
groot → grootst
ver → verst
duur → duurst
mooi → mooist

A spelling note: as no extra syllables are added along with -st, the spelling of a word is not changed.

If an adjective already has “s” in the end, then only -t is added:
dwaas (silly) → dwaast
fris (fresh) → frist

All superlative forms  add -e when they are in front of a noun:
Dit is de mooiste plaats van de stad.
This is the loveliest place of the city.

Zij koopt de duurste kleren.
Se buys the most expensive clothes.

We hebben het grootste huis.
We have the biggest house.

Superlatives add -e when used independently with the definite article:
Ze is de oudste van de drie.
She is the elder of the three.

Noun + verb + het + superlative:
Mijn vriend werkt het hardst(e).
My vriend work hardest.

Notice: Superlative gets an -e in the spoken language

Irregular superlatives:

(good) goed → beter → best
(much) veel → meer → mest
(little)  weinig → minder → minst
(…) graag → liever → liefst