Strong verbs form their past tense by changing the vowel in their stem — called ablaut — rather than adding a suffix. There are seven ablaut classes in Dutch, each with a characteristic vowel pattern. Class 1: ij → ee: rijden → reed, schrijven → schreef, blijven → bleef. Class 2: ie → oo: bieden → bood, vliegen → vloog, sluiten → sloot. Class 3: i/e + consonant cluster → o: zinken → zonk, beginnen → begon, vinden → vond.
Class 4: e → a: breken → brak, stelen → stal, spreken → sprak, nemen → nam. Class 5: e/i → a (with -en ending unchanged): geven → gaf, lezen → las, meten → mat. Class 6: a → oe: graven → groef, varen → voer, slaan → sloeg. Class 7 (reduplication class, now mostly vowel change): houden → hield, roepen → riep, lopen → liep, slapen → sliep. Learning the classes systematically helps you predict and remember irregular past forms.
The most practical approach is to learn the highest-frequency strong verbs immediately and rely on the class patterns for the rest. Prioritize: zijn/was, hebben/had, gaan/ging, komen/kwam, doen/deed, zien/zag, weten/wist, kunnen/kon, willen/wou/wilde, zeggen/zei, rijden/reed, schrijven/schreef. These appear in almost every conversation. Then gradually fill in the class patterns as you encounter new strong verbs in reading and listening.