Dutch spelling represents vowel length systematically. A long vowel in a closed syllable ending in a consonant is written with two letters: aa, ee, oo, uu. A long vowel in an open syllable ending in a vowel is written with one letter: a, e, o, u. When a word is inflected and the syllable structure changes, the spelling adjusts: dag (closed, short a) becomes dagen (open, long a — no doubling needed). Maan (closed, long aa) becomes manen (open, long a — one a dropped).
The consonant-doubling rule applies to preserve short vowel sounds. If a word ends in a short vowel plus single consonant and you add a suffix that would make the syllable open and thus the vowel long, you double the consonant: kat becomes katten (plural), rennen has hij rent (verb). This prevents the short a in kat from becoming a long a. The rule is: double the final consonant before adding a vowel-starting suffix if the preceding vowel is short.
Common challenges: the letter combination ij is always treated as a single vowel unit. Final devoicing — b sounds like p, d sounds like t, v sounds like f at the end of a word — explains why hond is spelled with d (honden in plural) but pronounced as hont. This spelling-pronunciation mismatch trips up learners who try to spell from sound alone. Always check spelling against the plural or inflected form to find the underlying form.