Type any adjective and see when it takes -e before a noun — and the one het-word case where it doesn't.
Almost every adjective adds -e before a noun. The only time it stays bare is a het-word with 'een' in the singular: een groot huis.
groot → grote, wit → witte, lief → lieve, vies → vieze. The tool applies the open/closed-syllable and f→v / s→z changes for you.
Loan colours (oranje, roze) and -en adjectives (open, houten, gebroken) never take -e. The tool flags them so you don't overcorrect.
An adjective in front of a noun almost always gets -e: de grote man, een grote man, grote mannen, het grote huis, grote huizen. The single exception is a het-word with an indefinite article (or no article) in the singular — there the adjective stays bare: een groot huis, geen groot probleem, lekker eten. After the verb (predicatively) it also stays bare: het huis is groot.
Because huis is a het-word and man is a de-word. With 'een', a het-word keeps the adjective bare (een groot huis), while a de-word still takes -e (een grote man). That single difference — de-word vs het-word after 'een' — is the whole rule most learners get wrong.
A few groups never inflect: loan colour words like oranje, roze, lila and beige; material adjectives ending in -en such as houten (wooden), gouden (golden), plastic; and past participles used as adjectives ending in -en like open, gebroken and tevreden. So it's 'een open deur', not 'een opene deur'.
Adding -e follows the normal Dutch spelling rules: a long double vowel becomes single (groot → grote), a short stressed vowel doubles the consonant (wit → witte, dik → dikke), final -f becomes -v (lief → lieve) and final -s becomes -z (vies → vieze, grijs → grijze). This tool does all of that automatically for any adjective you type.