Learn Dutch: Welk kaartje koop je? - Which ticket are you buying?

We use welk for het words (neuter words) and welke for de words and all plurals. The word kaartje is a diminutive (a "little" word ending in -je). In Dutch, all diminutives are automatically het words. Therefore, it has to be welk kaartje.

Why is the verb koop je instead of koopt je?

The verb here is kopen (to buy). Normally, the "you" form gets a -t added to the stem (as in jij koopt). However, there is a special rule: when je or jij comes directly after the verb, you drop that -t. That is why it becomes koop je.

Why does the verb koop come before je?

In Dutch questions, the verb often moves to the front. Here, the question phrase Welk kaartje takes up the first position. The verb must always sit in the second position, so koop comes next, forcing the subject je to move right behind it.

What exactly does kaartje mean here?