Mary comes to school unless she is ill.
The word modified by subordinating clause is comes.

Mary comes to school unless she is ill.
Adverb clause: unless she is ill

The man who wrote this new and popular book autographed copies of it at the bookstore.
Main clause: The man autographed copies of it at the bookstore.

The tree whose branches had died had to be cut down. Adjective clause: Whose branches had died; Modifies trees.