It's impossable to give all the rules for making plurals, but here are a
few guidelines:
Words that end on a syllable that includes a 'toneless -e' (like in de), usually get an -s:
de tafel - de tafels
de vader - de vaders
de haven - de havens
Words that end on a vocal usually get an -s with an apostrophe (') to keep the sound 'long':
de taxi - de taxi's
de agenda - de agenda's
de radio - de radio's
Other words usually get -en:
het boek (book) - de boeken
de krant (paper) - de kranten
de vrouw (women) - de vrouwen
Watch out with the spelling of vocals, though: if there's a 'long', double vocal it usually becomes a single one, followed by a single consonant, if it's a 'short, single vocal, it usually stays single, but followed by a doubled consonant:
de boot (boat) - de boten
het bot (bone) - de botten
de maan (moon) - de manen
de man (man) - de mannen
If there's an -f at the end, it usually becomes a -v unless the English version of the word has a '-ph' where the Dutch version has -f:
de brief (letter) - de brieven
de graaf (duke) - de graven
de paragraaf (paragraph) - de paragrafen
If there's an -s at the end, it often becomes a -z. There's no rule though, to tell for which words or sounds this happens, you just have to remember this for each word:
de kaars (candle) - de kaarsen
de laars (boot) - de laarzen
het kruis (cross) - de kruisen
het huis (house) - de huizen
Unfortunately, besides all this, there are a lot of hard to remember exeptions:
de dag (day) - de dagen
het dak (roof) - de daken
het glas (glas) - de glazen
het bedrag (amount) - de bedragen
het gat (hole) - de gaten
het graf (grave) - de graven (compare to 'de graaf'...)
het dal (valley) - de dalen
het vat (barrel) - de vaten
de het schip (ship) - de schepen
de smid (blacksmith) - de smeden
het lid (member) - de leden
de stad (town) - de steden
de oom (uncle)- de ooms
de broer (brother) - de broers
de musicus (musician) - de musici
de timmerman (carpenter) - de timmerlieden or de timmerlui (all professions on -man have -lieden or -lui for plural)
