Question

why do electrons enter the 4s sub shell before the 3d sub shell

Answer 1

3d sub shell has higher energy than 4s sub shell
electrons must be filled according to their increasing order of energy

Answer 2

In this question stability rule is applied(n+l). Here n is no. of shell and l is value of ----- s-0,p-1,d-2,f-3. So put the value. 4s - by n+l rule n-4 and l - 0 (4+0)=4. 3d by n+l rule n-3 , l-2 (3+2)=5 5>4 so 4s fill first