Diagramming sentences

So I'm sitting here trying to (once again) absorb into my being, when to use 'who' or 'whom.'  I hit this speed bump while reading the Mechanics chapter in Jack Hart's book, A Writer's Coach.

I stopped my reading after getting this sentence wrong, "Give the prize to whoever/whomever shows up first," (ans:  whoever   The "pronoun is the subject of the clause" not the "object of the preposition 'to'").  To better understand the reason, I went to a sentence diagramming website.  There is not only a website about diagramming sentences, there are many of them.  I chose the one by Gene Moutoux.

During this search to grok who/whom usage, my husband walked behind me and saw the sentence diagramming screen.  "You find the damndest websites," he said.  I guess websites about sentence diagramming aren't in his list of Favorites.