left subtree

 // c++
 if (curr->key > el)    // curr is a variable maintaining current node(already present in tree)
                        // with which comparison is taking place
                        // And, el is new key we have to insert
 {
     successor = curr;  // successor only contain value if new node
                        // goes in left subtree of curr node
     curr = curr->left; // go left subtree
 }