Ze latest newzes

So, after getting my "marching order" for Skopje, the first thing I did was plan my 5 weeks off preceding Skopje. I didn't really doubt that the Ambassador would authorize the leave of absence, since it means I'll be gone sooner.
I was right, and now things are looking like this:
1 June - leaving Ankara with ze boyz. 
9 june (to be confirmed but pretty sure) - going back to Ankara, because movers arrive on the 10th.
12 June - (or thereabouts) back to Vienna with extremely heavy suitcase.
20 June - (or thereabouts) 2 days in Skopje for house-hunting
4 July - leaving for Skopje

I'll have to pay for the trip to Ankara and back around 10 June myself, but it's definitely worth the money: Ze boyz won't have to undergo the moving stress. But it's tax deductible, as is the house hunting trip.

Conundrum

I seem to be a bit loquacious today, sorry. But this conundrum has been plaguing me for, oh, a loooong time, adn I haven't been able to solve it.

Is there any kind of rule for the use of the syllable in- or un- in the negation of adjectives and adverbs, or verbs for that matter? I think that words with Germanic roots are generally negated by sticking un- on them: undead, unfriendly, unloved, etc.etc. Which makes sense, because the German negation is un- as well.
But with words derived from Latin, there doesn't seem to be a rule: inhuman, but unacceptable. Inconspicuous but unpretentious. Implacable but unprepossessing.
My inability (and why on earh is it inability but unable????) to find out the secret pattern is quite unnerving (here we go again).

So, dear English-speaking friends, is there a rule, and if there is, ow does it work? I don't usually make many mistakes that way, but I'd love to know why I'm doing what 'm doing.