Subordinating conjunctions introduce dependent clauses and send the verb to the end. The most essential subordinating conjunctions: dat (that), omdat (because), als (if/when), toen (when — past single event), wanneer (when — general/repeated), hoewel (although), terwijl (while), voordat (before), nadat (after), zodat (so that), tenzij (unless), totdat (until). Each one triggers verb-final order in its clause.
The distinction between toen and wanneer/als is a classic learner trap. Toen is used for a specific past moment — it can only refer to a single completed event: “Toen ik kind was…” (When I was a child…). Wanneer and als are used for general, repeated, or future situations: “Als ik moe ben, ga ik vroeg naar bed” (When I’m tired, I go to bed early). Using wanneer for a past specific event — a common error — sounds unnatural to native speakers.
Stacking multiple subordinate clauses is common in Dutch and produces the complex verb clusters that learners find challenging. “Ik denk dat hij zei dat ze zou komen” (I think that he said that she would come). The verb clusters at the end of each subordinate clause contain all the verbs in the order: auxiliary + past participle or modal + infinitive. Writing out complex sentences and analyzing their verb clusters is the most effective way to internalize this pattern.