Often a main question is too complex to be answered in one go. That is why you split it into sub-questions. It makes the main question easier to manage.
Sub-questions always answer a specific part of the main question. Collectively, the sub-questions answer the main question. Questions that do not contribute towards answering the main question are not relevant.
Example
Your main question is:
“Which modifications to public transport are needed to ensure that the growing number of tourists in Amsterdam can move around more efficiently?”
You have now thought up a number of sub-questions:
These sub-questions certainly have their flaws. What is wrong with these sub-questions, and what would you do differently? If necessary, write down your suggestions!