noob question about weapons

newbie here currently playing a dk with uh and frost

when do we:
dual wield fast 1h weapons,
dual wield slow 1h weapons,
use a slow 2h weapon,
use a fast 2h weapon (does this exist?)

right now i am aware that frost dks dual wield slow 1h weapons, and uh dks use 2h weapons, yet i never really understood when we're supposed to be using which. if its about class and spec, i also want to know the reason behind such weapon choices.

sorry if my question seems confusing, i still have little knowledge about the game and other classes. thanks in advance!

When do we:
1. only if you're tanking as Frost because most defensive 1H weapons are fast speed
2. always if you're playing Frost because of higher base damage on slow weapons (compared to fast one) and all your melee hitting abilities scale with weapon damage (Obliterate, Frost Strike etc.)
3. if you play Unholy, because once again, your melees scale with weapon damage
4. never and yes

In addition, some classes use slow main hand and fast offhand, but frost uses both slow, because of talent that lets your abilities to strike with offhand weapon at the same time as main hand.