Breakdown of Welke sport doe jij?
jij
you
doen
to do
welke
which (for de-words and plural)
de sport
the sport
Questions & Answers about Welke sport doe jij?
Why is it welke here instead of welk?
You use welke because sport is a de word (de sport). In Dutch, you only use welk with het words.
Why is the verb doe missing its -t ending? I learned the jij form is jij doet.
You are right that the standard form is jij doet. However, whenever jij or je comes right after the verb, you drop the -t. Because this is a question, the verb moves to the front, turning jij doet into doe jij.
Could I use the unstressed je instead of jij in this sentence?
Absolutely. Welke sport doe je? is perfectly fine and very common. Using jij just adds a bit of emphasis, as if you are asking "Which sport do you do?" (perhaps right after you just shared what sport you play).
I learned the verb sporten (to play sports). Why not use that here?