Most Dutch nouns form their plural with -en or -s. The -en plural is most common and applies to most native Dutch words: boek becomes boeken, dag becomes dagen, kind becomes kinderen (irregular), stad becomes steden (with a vowel change). The -s plural applies to words ending in -el, -em, -en, -er, -ie and many loanwords: tafel becomes tafels, bezem becomes bezems, politie becomes polities. Words ending in unstressed -e generally take -n: vrouwen, mensen.
Spelling rules interact with plural formation in important ways. When adding -en to a closed syllable with a long vowel, the doubled consonant is dropped: dag becomes dagen rather than daggen. When a short vowel in an open syllable would become long, the consonant is doubled: kat becomes katten. These are the same rules that apply to verb conjugation and adjective inflection, so learning them once pays dividends across the whole grammar system.
Some plurals are irregular or unpredictable. Ei becomes eieren, kind becomes kinderen, been becomes either benen or beenderen depending on meaning (legs versus bones), stad becomes steden, schip becomes schepen, lid becomes leden. Diminutives always take -s in the plural: huisje becomes huisjes, kopje becomes kopjes. Learning irregular plurals is best done through extensive reading — encountering them in context fixes them in memory more effectively than rote lists.