Quantitative studies are studies that rely on counting and measurement...they rely on numbers. So if you want to do a quantitative study, you have to have some idea of what you are going to count or what you're going to measure.
Qualitative studies could be seen as any study that is not about counting, and not about applying numbers to things. It makes sense to look at this as an examination of the qualities of things, but qualitative studies can also operate in the same logical context as quantitative studies, if the qualitative study is viewed as an opportunity to examine possibilities (see my previous post on this).
The questions asked by quantitative studies and by qualitative studies are different.
Here are some quantitative questions.
"How many are there?"
"How often does this occur?"
"Are these measurements correlated?"
"Which occurs more frequently?"
"Is the rate of occurrence stable or changing?"
"How do these groups differ for a given measurement?"
"What are the likely results of an action, and what are the likelihoods/probabilities?"
"What is it like to do X?"
"What are the experiences of group Y?"
"What are the perceptions of group Y?"
"What are the characteristics of group Y?"
"What has occurred?"
"What are possible consequences of that action?"
"What are different ways that Z can be interpreted?"