Letter Placement's Affect on Sound

Good day friends, I have a small quandary that I assume most of you can quickly address. In lesson 2.1.1 it is stated that Alef in the middle of a word carries the 'A' sound, as appose to the 'a' sound. Yet later the word حمار is pronounced 7omar instead of as 7omAr. Is this an exception, or a certain rule I do not currently know? 

I'm not expert, but I've found that the alef can be pronounced several of different ways and I don't really see a pattern.  شاي , جواز , شارع , طماطم ... and I'm sure I'm probably missing some.   I typically just guess "A as in apple" unless I know otherwise or if it's followed by a ي or ى.

Hi dragoonmaster101 and Ph0t0n,
Sorry if we missed your question earlier. You're right, he letter alif can be pronounced in either a "thick" way, as in

or a "light" way, as in

. It usually depends on the letter that comes before the alif, but of course there are some exceptions, and the rules are different than those that apply for MSA. In general, the letters that make the alif sound light are 
ت - ث - ج - خ - د - ذ - ز - س - ش - ع - غ - ف - ك - ل - ي
The letters that make the alif sound heavy or thick are 
ب - ح - ر - ص - ض - ط - ظ - ق - م - ن - هـ - و
It is difficult to memorize them, but you will get used to it by practice.

Cool, thanks noha!