Warning! Side effects of asking the following questions might include arguing, feeling hurt or irritated with your partner, and result in more questions than answers. You should not ask these questions when your partner is hungry, sick, or already in a bad mood.
And with that out of the way, let's continue. All questions are equally important, so I will list them in no particular order.
1. Do you want children?
Although the answer to this question, may change over time, it is crucial to discuss whether or not you want children because children are a huge stressor for marriages. Children bring on stress and can trigger the ugliest side of ourselves, so don't assume that your partner does want children. Don't assume that you can change your partner's belief about having children either. Having this discussion before your marriage, will ensure that you both have entered the marriage with the knowledge of one another's expectations about having children.
2. What financial responsibilities/commitments do you have now?
Money is a very scary topic to talk about. Most people would rather not talk about it, but it will come up even if you avoid it. This question is important to ask and know fully before marriage because you will inherit your partner's financial problems once you get married. Knowing how much debt they have can give you insight about their spending and saving habits, and how they prioritize paying their bills. It can also let you know if they help out their family financially or what not. Again don't assume that once you're married, your partner will immediately change their financial priorities to fit yours. They've been doing what they have for many years, and it's a habit likely to continue.
3. What religion will we practice in our family?
Oh so important, but a touchy subject to discuss. Whether you practice the same or different religion, I cannot stress the importance of talking about this before marriage. Usually I recommend couples having the same faith, but in some cases couples can make it work with two faiths. However, this question is about practicing faith in your family, which includes your future children. So even if you both have your own faith, will you children as well? And which faith would they practice?
4. How will we spend the holidays?
If you don't celebrate any holidays, then this question is irrelevant to you. But perhaps, discussing how often you will visit each family is more of the issue. Especially if you do not live in town with them. Having this discussion will prevent future hurt or defensiveness about spending too much or too little time with one side of the family.
5. What are your beliefs about divorce?
This question will probably catch you off guard because it's seems silly to ask about divorce if you're not even married yet. However, this question is actually more about assessing your partner's beliefs about issues that would a deal breaker. Your partner's convictions about marriage can also indicate that they are more likely to weather the marriage storms. It wouldn't hurt if you also ask yourself this question.