Pronouncing «е»

[ugly-boy.livejournal.com]

How do you know when 'е' is pronounced йэ and when it's simply pronounced э? Romanization schemes don't always help—some always write it as je/ye and others always write it as e preferring to write э as è. For instance, I know that at the beginning of a word 'е' is always pronounced йэ but I'm not sure about the middle and end of a word. I know that нет = нйэт but what about не? I always assumed that привет was pronounced 'привэт' until a Russian woman at work greeted me with a cheery Privjet! the other day. What's the deal?

Also, can я, ё, and ю ever be pronounced а, о, and у? And why isn't there a йи sound like there is in Ukranian (Її)?