First of all, we need to understand the meaning of this problem in order to convert this subtraction operation into a simpler subtraction operation.

Question 1 (upper left): fill in 8 at the far left, 10 at the right, 1 at the bottom and 9 at the top;

Question 2 (upper right): Fill in 6 on the far left, 10 on the right, 1 below and 7 above;

Question 3 (bottom left): Fill in 5 on the far left, 4 on the right and 6 on the top;

Question 4 (bottom right): Fill in 10 on the left, 1 on the right, 3 at the bottom of the middle two and 4 at the top.