Dr. Yaling Zheng
If R4C4 Iis number 3, R4C7 has to be number 4 (step 1), and then R5C9 has to be number 3 (step 2), and then R3C9 has to be number 4 (step 3), and then R3C3 has to be number 8, and then R1C3 has to be number 4.
Therefore, no matter what number R4C4 is, R1C3 will be number 4. This is a great example of using forcing chain to determine a certain number for an empty square. Forcing chain is an advanced method for expert Sudoku solvers. It takes time and patience to find forcing chain patterns. And I only recommend you use this method if all other simpler methods do not work.