A diphthong is a vowel sound that glides from one position to another within a single syllable. Dutch has several diphthongs worth systematic attention. IJ/EI (written as ij or ei): pronounced like English “eye” — mijn, wijn, klein, reis, trein. UI: this is the most challenging Dutch diphthong — start with lips rounded as if for “oo” then move toward “ay” — huis, muis, buiten, trui, tuin. OU/AU: similar to English “ow” in “now” — oud, out, blauw, grouw, auto.
OEI: starts with the “oo” of “food” and glides toward “ee” — roeien, moeite, groeit. OI: appears in some loanwords — stoïcijns, aloïne. The difference between ui and ij is crucial: mijn (my) vs. muin (not a word, but the sound difference is clear). And wijn (wine) vs. wuin — practicing these pairs trains your mouth to produce distinct sounds that native speakers easily distinguish.
The most common learner error is substituting English “oy” for Dutch UI — huis is not “hoys” or “howss.” The Dutch UI requires specific lip rounding that English does not use in this position. Daily practice with five UI words — touching your lips to make sure they are rounded — for two weeks produces significant improvement. The sounds ij, ui, and ou together account for a large proportion of Dutch pronunciation errors among English speakers and are the highest-priority diphthongs to master.