Dutch has a rich vowel system with important long-short distinctions. Short vowels: a as in pat (kat — cat), e as in pet (bed — bed), i as in pit (bit — bite), o as in pot (bom — bomb), u as in put (bus — bus). Long vowels: aa as in father (maan — moon), ee as in German See (been — leg/bone), oo as in so (boot — boat), uu as in French lune (muur — wall). The long vowels are generally tense and pure — they do not diphthongise the way English long vowels do.
The open/closed syllable rule determines vowel spelling in Dutch. In an open syllable (ending in a vowel), a single vowel letter represents the long sound: le-ven (to live, long e). In a closed syllable (ending in a consonant), a single vowel letter represents the short sound: lev-end (living, short e). When a stem needs a long vowel in a closed syllable, the vowel is doubled: been (bone), maan (moon), boot (boat). This rule explains the famous doubling/dropping in conjugation: ik loop → we lopen (one o becomes two syllables, open).
Special vowels: eu (as in neus — nose) is a rounded front vowel — lips rounded for oo, tongue position for ee. The ij/ei diphthong (as in zijn, trein) starts with a short e and glides to a j sound. The ui diphthong (as in huis, buiten) is uniquely Dutch — a rounded front vowel gliding to a schwa. The ou/au diphthong (as in oud, blauw) is like English ow in how. Mastering these vowels is the single biggest factor in sounding authentically Dutch.