Sometimes when you are subtracting mixed numbers, the numerator in the top fraction is smaller than the amount you are trying to take away. Just as you would borrow from "next door" when subtracting whole numbers, you will need to borrow from the whole number to complete the subtraction.
See the example below:
You may be asked to subtract a fraction from a whole number. In this case, you would borrow one whole from the whole number and add back a fraction worth one whole. To make things easier on yourself, create the "whole" using a common denominator for the problem you are working with.
In the example below, you are subtracting 1/4, so it is easier if the "one whole" you create is 4/4 so you have a common denominator and can subtract.