Borrow 1. When 10 is not enough to reduce the single digit, borrow 1 from the last digit to reduce it, and this borrowed 1 is 10 if the decimal digit is borrowed, 100 if the decimal digit is borrowed, and so on.

For example: 16-8=8, and 6 is not enough to subtract 8. Borrowing 1 from ten digits means that 10 minus 8 equals 2, and the single digit itself has a 6, and 6 plus this 2 means that the final answer equals 8.