Learn Hardcore Dutch: Het optreden waarvoor we kaartjes kochten begint om acht uur. - The performance for which we bought tickets starts at eight o'clock.

Dutch nouns are either de-words or het-words. Optreden is a het-word, so you use het optreden. Unfortunately you have to learn many of these by heart.

Why is kochten placed at the end of the clause waarvoor we kaartjes kochten?

That clause is a subordinate (relative) clause. In Dutch subordinate clauses the conjugated verb goes to the very end:

Main clause: Het optreden begint om acht uur.
Relative clause: …waarvoor we kaartjes kochten.

Why does the sentence use waarvoor instead of dat or wat?

Because in Dutch you need a relative pronoun that matches the required preposition. You bought tickets voor something, so you combine waar (where/for which) + voor = waarvoor. You cannot use dat or wat here.

Could you split waarvoor into waar … voor like in some other Dutch sentences?

Yes, in informal or spoken Dutch you often say: “Het optreden waar we voor kaartjes kochten begint om acht uur.” Both versions are correct:

waarvoor we kaartjes kochten (more formal/standard)
waar we voor kaartjes kochten (more colloquial)

Why is there no comma before begint?

In Dutch you do not separate a restrictive relative clause with commas. The phrase waarvoor we kaartjes kochten specifies exactly which performance you mean, so no commas are used.

What does kaartjes mean, and why not just kaarten?

Kaartjes is the diminutive plural of kaart and means tickets (often used for concerts, theater, etc.). You could say kaarten, but kaartjes sounds more natural when talking about event tickets.

Why is the time expressed as om acht uur rather than just acht uur?

Dutch uses the preposition om to talk about clock times:

om acht uur = at eight o’clock Without om it would sound incomplete or like you’re naming a duration rather than a point in time.