I never bothered really with enchamnting much but got to the point in the game where i just pumped out some daggers and rings to get up to 100. Now i want to start enchanting for personal use and something seems weird. I says that when you disenchant something to learn the enchantment you learn the "base enchantment". Ok...makes sense. Also, obviously, you cant put a damage enchantment on armor, so i understand that. And shield enchantments on armor or weapons...makes sense

However, i have an enchantment that reduces destruction spell cost and increases magicka regen. I learned that from disenchanting some sort of robe or armor. Now i go to enchant some armor pieces with it and it will only go on certain items. It will let me put in on my armor but not my helm?? I know the enchanment that is just reduce destruction spell cost, and it will let me put that on my helm, but not the combined enchantment.

Now, i have a orcish helm with the reduce destruction spell cost and increases magicka regen enchanment on it that i found. It wont let me disenchant that helm because i supposedly "already know that enchantment" but when i go to enchant a helm with it it wont let me apply it to a helm. How can i already know the enchnment that is on a helm that i have but not be able to put it on a helm

I am totally confused as to how the game decides what enchantments can go where. Obviously, the enchanment that is on the helm i have can go on a helm, but, it wont let me learn it cause i already know it, but, it wont let me put it on a helm.

Can someone explain this game mechanic?

Also, resistance to magic can only go on jewelry? I'd swear i disenchanted a pair of boots to learn this but now it will only let me enchant jewelry with it.

Anything that can help clarify this is greatly appreciated!!!

Also xbox...so no console commands

http://elderscrolls.wikia.com/wiki/Enchanting_(Skyrim)

Under the header ''Apparel enchantments'' is a list that shows you what enchantments can be put on what item. Some of the items you noted should not be enchantable in that way, but perhaps they are unique items. Sometimes items with certain enchantments only appear like that in dungeons, and can't be crafted that way.