Type any Dutch adjective and get its -er and -st forms, with the spelling rule. groot → groter → grootst.
Type any adjective and get both the comparative (-er) and superlative (-st), with the spelling change spelled out.
groot → groter (not grooter), lief → liever, vies → viezer, dik → dikker. The engine applies the open/closed-syllable and f→v / s→z rules for you.
goed → beter → best, veel → meer → meest, weinig → minder → minst, graag → liever → liefst. Learn these by heart; the rest follow the rules.
For most adjectives you add -er for the comparative and -st for the superlative: klein → kleiner → kleinst. Spelling changes follow the normal rules: a long double vowel becomes single when the syllable opens (groot → groter), a short stressed vowel doubles the consonant (dik → dikker), final -f becomes -v (lief → liever) and final -s becomes -z (vies → viezer). Adjectives ending in -r add -der (duur → duurder), and after -s the superlative adds only -t (vies → viest).
The four you must memorise are goed → beter → best (good), veel → meer → meest (much/many), weinig → minder → minst (little/few) and graag → liever → liefst (gladly). These don't follow the -er/-st pattern.
The comparative is used with 'dan' (than): Amsterdam is groter dan Utrecht. The superlative usually takes 'het' and an -e ending: Rotterdam heeft de grootste haven; hij loopt het snelst. In front of a noun both add -e: een groter huis, het grootste huis.
Yes — unlike English, Dutch almost always uses the -er/-st endings even for longer adjectives: interessant → interessanter → interessantst, belangrijk → belangrijker → belangrijkst. You rarely need 'meer' or 'meest' the way English uses 'more' and 'most'.