What information do you need to answer your main research question?
The main research question cannot be answered in one go.
That is why you formulate a number of sub-questions based on your main research question. Sub-questions allow you to divide your complex main research question into smaller subjects. Each sub-question contributes to answering your main research question. Once you have answered all the sub-questions, it should be possible to answer your main research question as well.
Tip:
Sub-questions: example
Below are a number of examples of sub-questions:
"What is the definition of obesity?"
This is not a good sub-question, because sub-questions are not about formulating definitions. The sub-question must always be necessary to answer your main research question.
"Does food supply have an effect on obesity?"
This is not a good sub-question, because it is a closed question. Sub-questions must be formulated in an open manner, so they cannot be answered with a simple 'yes' or 'no'.
"What measures are being taken in school canteens to combat obesity among students?"
"What influences students' food choices?"
These are good sub-questions, because they are less complex than the main research question, but more specific and help to answer the main research question.