I'm actually Spectral Dancer sub Sagitarius sub Hell Knight, on the Spectral Dancer i got passive m.def lvl 4, Counter critical and Counter p.def/m.def.
So if i choose to be Hell Knight as main, will i keep all those subskills i've learned or will they be reseted ? same for transform skills (aurabird falcon, heretic etc..) i've try to look on boards but got a low provider connection and can't find answer.
You keep transformation skills and all other skills on each of the classes (i.e. if you learned a transformation skill on your old main, you will have that skill on the new subclass, not on the new main). All the subclass certificate skill will be reset and you have to learn them again, from the options your new subclasses give. So in your case, with the Hell Knight main, you will have to choose from the options the SD (enchanter sub) and archer (rogue sub) give. You can get the mdef lvl 4 again, you can get the counter crit from the archer, but you cannot get the knight UD (instead you might want to get chance celestial shield from the SD sub).
if you also get the skill books (i.e divine transformation) and then wait until the sub to main is done, and try to use the book on your "new main". the book(s) will have been wiped also during the sub to main process