this might have been better places in the questions forum, but...
you start out by picking a job, it's not permanent, it just basicly decides what job you start out as, and what equipment you start with.
then, once you get in the actual game, at any time you're able, you can go to your mog house and change your job to whatever job you have available at the time. you keep all your lvls in your previous job, and can switch back the same way any time you want at your mog house, with no penalties.
to set a subjob, you first have to be lvl 18 and complete the quest in selbina or... muahuara?(sp? sorry, haven't been there. :p).
after you complete that quest you can then go back to your mog house and set your subjob to be whatever you want, but you have to have at least lvled that class a little bit to be able to subjob it.
...annnnnd i guess your main question was could you have a sub thf lvl 30 and a drk main? yes, you can, but to have a lvl 30 sub thf, your drk main would have to be at least lvl 60, because regardless of what lvl you lvled your sub to before you set it to subjob, the game sets your subjob to a "temporary cap" of half what your main is. your subjob's temporary cap will never be higher than half that of what your main job lvl is. then if you choose to set your thf back to your main, it would be back up to it's full lvl, if it was higher than 30 originally, that is.
hope that makes sense, and answered your question.
