Dutch does not have a progressive tense the way English does — there is no direct equivalent of “I am working.” Instead, Dutch uses zijn + aan het + infinitive to express an ongoing action at the moment of speaking: “Ik ben aan het werken” (I am working), “Hij is aan het koken” (He is cooking), “Zij zijn aan het studeren” (They are studying). This construction is informal and very common in spoken Dutch.
In subordinate clauses, the construction stays intact and clusters at the end: “…omdat ik aan het werken ben.” Note that zijn is conjugated as usual, but the entire aan het + infinitive unit moves to the end together. This is different from the ordinary verb + infinitive cluster where only the infinitive goes to the end. The aan het unit is inseparable in this construction.
Two alternative ways to express ongoing actions: zitten/staan/liggen/lopen + te + infinitive add information about posture or manner: “Ze zit te lezen” (She is sitting reading), “Hij staat te wachten” (He is standing waiting). These are very natural in Dutch and add a vivid sense of what the person is physically doing. The te construction here is slightly different from the standard te + infinitive — it follows the posture verb and expresses simultaneous action.