Mandarin Chinese pronunciation (pinyin)?

Hi, I have a number of questions about Mandarin pinyin pronunciation:

1. How do you know when "i" in pinyin should be pronounced "uh" or "ee" or "er" as in the English word "fur" (examples of usage in chi and shi)? I've heard it as all these ways and am very confused as to which is correct, keeping in mind I am aiming for the Beijing accent.

2. In the pinyin "e", how does one know if it is to be pronounced as the English sound "uh" or "eh"? I have heard "ye" as both "yeh" and "yuh".

3. Does it really make a huge difference if you give your "ch" and "zh" different forces of air, like I know some of the sounds have more air than others, like "p" and "b"...?

4. Which region's/city's accent is the most understood throughout the PRC?

5. "Eng"- pronounced like "ung" or "ang" in the english word "hang"? How do I know which is right? Like for example, cheng2 chi2

I know that if I heard the word before I should know how to spell it in pinyin the problem is I'm hearing so many different ways to say things! Please help! Xie Xie!!
So are there exceptions to the pinyin "e"? Such as in "de" in "wo de"? Isn't that pronounced "duh"?

Thanks for the info on pronunciation variations though...I'm sure it's similar to Spanish, with Castilians having a lisp and the Argentines with their pronunciation of "ll".

How do you say “girl” and “boy” in basic pinyin chinese?

Is girl: yi ge nu hai er [no accents included]
boy: yi ge nan hai er

But I don't understand why are there so many words! Is this common? Can you translate each word into a english word? Thanks

PS: I know nothing about chinese and just started learning using Rosetta Stone which is incredibly vague with the pictures
awesome! thanks for the help. I understand now.

Chinese word pronunciation help?

I'm a beginning student of Mandarin Chinese and am running into a disagreement with a friend over proper pronunciation. I always thought the word hen3 很 rhymes with the English word "fun," with the e pronounced like a deep "uh" in the throat. However my buddy thinks it sounds like the English word "hen." What is the correct way? I'm mainly interested in how it would be properly said in Beijing. Thanks in advance.