Monthly housing expenses include the principle, interest, taxes, and insurance. The last factor is credit. Now if you add the monthly housing expense and the monthly consumer debt you will come up with the total debt to income ratio. Some lenders can work with higher DI's but, the rate and LTV will suffer. Like I said, these are just desired ratios, they are not set in stone.
Different lenders have different guidelines to fit consumer needs.
Well that all depends on the three factors put together. So if you are the buyer you can figure out about where you stand in credit. Here is a basic chart of principal and interest for a 30 year amortization of a loan. Good luck and don't worry about me collecting your homework.