If you know what inorder is then predecessor / successor is too easy
e.g inorder of a BST is increasing sorted order
3 5 8 12 15 21 25 27 31 35
if u want to find inorder successor of 15
u can see it is 21
Now how to find it in BST inorder successor of 15
pick up right subtree of 15 and find minimum element there which is 21
And to minimum element in BST you have to travel to leftmost branch upto leave i.e go left left left upto leave
Overall Conclusion : 1. Pick right subtree
2. then go left left left ..... find minimum element in that subtree which is leftmost element
If you understand this predecessor is just reverse of it
lemme know if you still have problem