Your dh is correct about AN, but the H is also pronounced. “An historic landmark.”
Sadly, this and many other rules of speech are being changed as keypads become the main method of communication among so many illiterates. This H issue is very controversial. I think the major newspapers now use only “A” insread of AN, who knows what their point is!?
Keen the AN!
Keep the AN!
Defend the articles
Like a MAN!