@pjoa09 J means that your decision making function is extroverted (Te for INTJ, Fe for ISFJ). If you are an introvert, your dominant function has to be introverted. So that will be the other middle letter: N, S. So we get
Ni-Te for INTJ and Si-Fe for ISFJ.
The inferior function is the opposite of the dominant, as in... it's the other half of the dimension of the dominant and with opposite orientation.
--> Ni - Te- .....- Se for INTJ
--> Si - Fe - ..... Ne for ISFJ
Then fill in the tertiary: it's whatever is missing and has the same orientation as the dominant
Ni - Te - Fi - Se for INTJ
Si - Fe - Te - Ne for ISFJ
There is some debate as to the orientation of the tertiary, but this is the most common version.
For a perceiver, it's the information gathering function that's extroverted.
So that gives us Ne for an INTP. But because an INTP is an introvert Ne can't be his dominant function. So his dominant is the other middle letter
--> Ti - Ne - ......- Fe ----> Ti-Ne-Si-Fe