Pronunciation?

How can you tell whether the "a" in an Arabic word is going to be pronounced "eh" or "ah"? Also, when breaking down "il 7amdulillah," Hany says the last "ah" in the phrase as "ah," (part of Allah), but when he puts it back together it goes back to being pronounced "eh". o.0

Classical Arabic, the Arabic used in the Qur'an, from which Egyptian Arabic and pretty much every dialect of Arabic originated from, only had 3 vowels - a, i, and u. Each vowel could be either short or long. If you read Qur'an you will find no other vowel besides a, i, or u. However, like most languages, Egyptian Arabic has become quite colloquial, and thus each vowel can be realized as a few different phonemes. For example, the "i" can sound like the "ee" in "seed" or the "i" in "pit", both short and long. The "u" can be realized as the "oo" in "soon", the "u" in "but", and even the "o" in "boat". Believe it or not, there was a time when the English "a" only ever sounded like the "a" in "father". However, over hundreds of years, the "a" has taken on many different forms. The same applies to our other vowels - i, o, u, e/... Similarly, the Arabic Alif ("ah") has developed a few different realizations. You can have the "a" in father, the "a" in "bat", the "e" in "bet" and I'm not sure, but I think it can also sound like the "o" in "boat" as well. It can also sound like the "ee" in "seed". For example, the word ال, which means "the", was originally pronounced as "al", but has since developed into a sound somewhere between "EHL" and "EEL". So when emphasis is put on an alif, it sounds like "ah", usually, as in the word الله (Allah), however, when "Allah" is stringed onto the word الحمد (alhamd), which means "praise" or "all praise", the beginning of the whole word "alhamdulillah" is where the emphasis lies, so the final "a" is reduced to a sound more like "eh". This is why Hany pronounced the broken down الله as "allah", but when put back together the last vowel is said more lazily, as we also do with unstressed vowels in English, ie: the second vowel in "cigar" (sih-GAR), but then we say "cigarette" (sih-guh-RETT). If you ever listen to Qur'an, particularly the first verse of the first Surah (al-Fatiha), you will find all the vowels pronounced as they are written, because it is non-dialectal Classical Arabic. So I think that generally, an Alif is reduced to an "eh" or "ih" when unstressed, but when stressed, it takes on its original sound. Hence "allAH", but "ilhamdulillEH".

Thanks Hassan for the detailed explanation :)