Strong verbs (sterke werkwoorden) form their simple past by an internal vowel change rather than by adding -te or -de. This is the same Germanic system as English sing-sang-sung or drive-drove-driven. Dutch examples: rijden (to drive) becomes reed, schrijven (to write) becomes schreef, lopen (to walk or run) becomes liep, vinden (to find) becomes vond, binden (to bind) becomes bond. The past participle of strong verbs takes ge- plus stem plus -en: gereden, geschreven, gelopen, gevonden.
Strong verbs are grouped by the vowel change they undergo, called ablaut classes. Common patterns: ij becomes ee (rijden becomes reed), ie becomes oo (vliegen becomes vloog), ei becomes ee in writing (schrijven becomes schreef), i becomes o (vinden becomes vond), e becomes a (eten becomes at), ui becomes oo (ruiken becomes rook). Recognising these patterns allows you to make educated guesses about unfamiliar strong verbs encountered in reading.
The most frequently used Dutch verbs are strong: zijn (was, geweest), hebben (had, gehad), gaan (ging, gegaan), komen (kwam, gekomen), zien (zag, gezien), geven (gaf, gegeven), nemen (nam, genomen), doen (deed, gedaan). Learn these thoroughly first. Then expand to the next tier: lopen, rijden, schrijven, vinden, houden, staan, liggen, zitten. A dedicated strong-verb drill session once a week builds mastery faster than passive exposure alone.