Toen ik de fietsband plakte, merkte ik een tweede lek op.

Breakdown of Toen ik de fietsband plakte, merkte ik een tweede lek op.

ik
I
een
a, an
toen
when
tweede
second
de fietsband
the bike tire
plakken
to patch
opmerken
to notice
het lek
the leak
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Questions & Answers about Toen ik de fietsband plakte, merkte ik een tweede lek op.

What is the function of Toen in this sentence, and why does plakte appear at the end of the first clause?
Toen is a subordinating conjunction meaning when (referring to a single event in the past). In Dutch, subordinating conjunctions push the finite verb to the end of their clause. That’s why plakte (past tense of plakken, “to patch”) comes after de fietsband.
Why does the main clause start with merkte and not ik merkte?
After a subordinate clause at the very beginning, the following main clause still obeys Dutch V2 word order. The finite verb (merkte) must come directly after the comma, so the subject (ik) moves to third place.
In merkte ik een tweede lek op, why is op at the end, and is it related to merken?
Yes. The verb here is the separable verb opmerken (“to notice”). In main clauses, the prefix (op) detaches and moves to the end. So you get merkte (finite verb) in second position and op at the very end.
Why does the sentence use the simple past plakte and merkte instead of the present perfect?
Dutch speakers use both simple past (imperfectum) and present perfect (perfectum). In spoken informal Dutch the perfect is most common for past events, but the simple past is widely used in narratives, written texts and in the northern Netherlands. Here, the simple past gives a straight storytelling feel.
What’s the nuance between saying een tweede lek and nog een lek?

nog een lek = one more leak, focusing on an additional puncture.
een tweede lek = a second puncture, emphasising the ordinal sequence (there was a first, now a second).

Could I replace toen with terwijl: Terwijl ik de fietsband plakte, merkte ik een tweede lek op?
You can, but with a nuance. Terwijl implies two actions happening continuously at the same time; toen points to a specific moment in the past. If you want to stress simultaneity, use terwijl; if you describe one action leading up to the moment you noticed, stick with toen.
Can I write the sentence with the main clause first, like Ik merkte een tweede lek op toen ik de fietsband plakte?

Yes. Placing the main clause first removes the need for inversion after a subordinate clause:
Ik merkte een tweede lek op toen ik de fietsband plakte.

Does fietsband here mean the tire or the inner tube, and could I say binnenband instead?
In everyday Dutch, fietsband often refers loosely to the whole “flat‐tire situation” (tire + tube). The hole is actually in the binnenband (inner tube), which you patch. If you want to be precise, use binnenband, but it’s common to just say fietsband when talking about fixing a flat.