One mysterious thing about Dovahzul is its use of the apostrophe.  It is seen in words like thu'um, zu'u, and su'um.  Thus far our explanation for them has been to pronounce these as "thoo-um", "zoo-uh", etc. although they are usually shortened to "thoom" or "zoo".

However, looking closely, the apostrophe actually doesn't exist in Dovahzul at all.  In-game, words such as thu'um are writen in the Dovahzul alphabet as thuum.  This means that the apostrophes only exist in the English spelling of the words, not the words themselves.

With this in mind, what is the apostrophe for?  Does it affect pronunciation at all?

I think that what does affect pronunciation is the use of two times the letter "u" instead of the use of the letter "uu", because its important to remember that dovahzul is a lenguage, completly independent from english, and that in dovahzul we have different letters that dont exist in english. Think of it well... What is the use of having a letter for "uu" if its not different at all from "u"? I think that they have different pronuntiation and that the use of those letters diffear in pronuntiation. Also, while writing in english, we have no letter for "uu", the guys from skyrim found the apostrothe the easier way to create a readable difference in english.

Good thread, very well noted, i think it deserve some atention.

What's interesting about that though is that there are words that have "uu" but don't have apostrophes. The canon one that comes to mind is "Gravuun".  So, all words with apostrophes in English are spelled with "uu" in Dovahzul, but not all words spelled with "uu" in Dovahzul have an apostrophe in English.

well, thats a problem... So, even when they use double u they should use the letter for "uu"?

Correct, as of yet there's never an occurence of a word using two "u"s instead of one "uu".