We learned them threw the training of our class style
Youre choosing to play a fantasy based MMO. Make up your own back story.
There really is no particular reason set in stone. Basic MMO mechanics dude. Just RP a reason why. Just don't go thinking too much into stuff.
this is either a horrible rendition of 'how do i shot web' or you just awoke from a frozen time warp. literally no other explanation, but if i had to put real money on it im goin with teh cryogenics angle
Molag Bal took our souls for goodness sake! Is it so wrong that we should learn to throw fireballs in return? Maybe we secretly have use of a spare daedra soul that was going round?
Your character was born with the ability to learn them, and when you get skill points, you can choose to learn the skills.
Interesting question. I guess you mean in comparison to how you must "buy spells and learn them" by reading spell tomes or by talking to a mage/spellmaker in previous Elderscrolls games
Consider the fact that most players in previous ES games have spells that they start off with, yet it is never mentioned how they attained those spells. Now apply this curiosity to an onion like class -- as you peel the layers back, you'll find that the base spells/skill trees you have are rooted within eachother, so the more you develop these skills the more advanced they become.
I consider it to be a natural form of learning.
I like to think the player character learned their skills in some order or school initially. And as we level with them, certain abilities are 'remembered', restoring a bit of who they were before Molag Bal murderized them.
Of course that's only when i feel the need to justify the awesomeness of my character beyond the fact that its a fantasy game in a world of magic, gods, and practitioners of epic badassery.
Because the stick of truth allows us to have these powers.
http://buttonsmashingreviews.files.wordpress.com/2012/07/aliens.png
Breathing fire? Brewers on tamriel receive no formal training, nor do they comply with even basic health & safety protocols (have you seen what they put in that stuff? o_O). If there's a question then I'd suggest: "why don't more people spontaneously combust?".
Marking a target for death? if you seriously want to know, please ask around for the 'dark brotherhood', but... don't mention my name, thanks.
Crystal shards? I'd have to refer you to those alchemy tables. Mixing alchemy ingredients is "not rocket science", or at least it's not supposed to be... (I'd suggest NOT placing those vials in your trouser pockets - crystal shards can be exceedingly painful in certain anatomical regions)