The proper time to borrow is when you need to subtract the two fractions but need to use the whole number to do so or when the number is simply big. For example 12 2/11 - 4 9/11. I will explain to you by using pictures, step by step.
The proper time to make a fraction into an improper fraction is when the first fraction cannot be subtracted by the second fraction. For example, 6 1/2 - 1 3/4.
Now that we've subtracted the whole numbers, we need to subtract the fractions. In order to do that, we have to firstly find the common denominator. Both of the denominators fit into 4 equally, so our equivalent fractions are now 2/4 and 3/4. 3/4 stays the same because the denominator is already 4.
* To get an equivalent fraction, you have to see how many times the first denominator goes into the new denominator. Then you have to multiply how many times by the numerator. (E.g. 1/2: 4 is the new denominator. 2x4=2. 2x1=2. New fraction is 2/4.)
After you've changed the fractions, you still can't subtract but that's how it's supposed to be. So the next thing you need to do is multiply the whole number with the denominator and add the numerator (just like when you're borrowing).
Finally, all you have to do is subtract the fractions and you'll get the answer.