How do I explain the following to my players?

[carborundum]

Heh!

But yeah, they kind of need to just accept that that's how D&D works: with a good enough roll, anyone can hurt anyone regardless of their armor.

It doesn't have to hurt him - just stagger him, knock him back, try to trip him and really scare him... Less hitpoints is also him thinking less about killing the wolf and more about not ending up floundering on his back in plate armour while it looks for an exposed bit of flesh to bite.

Every time the wolf almost gets him, he loses the fight a little.

 

[Timeboxer]

I usually use the phrase "You feel winded" in these situations. As in: "The wolf lunges to bite you and you just barely leap out of the way, but you feel awfully winded."

 

[Moon-Lancer]

in midevil sword fights when both combatents had fullplate, They needed to resort to special combat called half sword, otherwise its a pretty futile endeavor.

Combatants where pretty safe in fullplate. Its when your dragged off your horse and the presents get a free shot at all your weak spots that you really need to worry.