1. When will he bake a cake?
2. When will the game starts?
3. What did he builds?
I think no. 3 is incorrect.
Do I always have to follow subject-verb agreement even in interrogative sentence?
The game will start when. Will has the same form for all persons. Will, will, will, ...
When (the 3rd person singular of will = will) will the game (game = 3rd person singular noun, subject) start.
Agreement means this:
game = 3rd person singular noun, subject
the 3rd person singular of will = will
...will the game...