you need to regroup when the bottom number is bigger than the top number
Yes, you do 73 -35 ___ 38
No, you need a miracle! 73 does not equal 35 and no amount of regrouping will alter that.
If the total in any column exceeds nine, you will have to regroup.
With addition, you need to regroup in the event that there are more than 10 ones or 10 tens. In a subtraction, you must regroup whenever there are insufficient ones or tens to subtract.