I'm a Singapore citizen and tax payer. While I know the basics of why
income are taxable, I'm confused how it all work together when it
comes to getting a foreign income from USA.

The kind of incomes I'm referring to are two types:
A) Dividend payout on the company stocks I purchase via an online USA broker.
B) Commission payout when I participate in e-commerce as an affiliate
for US companies selling their products/services online to global
market (not only to US residents)

Both type of activities require me to complete a W-8BEN form to
declare that I'm a foreigner, which I did.

Here are the specifics I like to find out:
1) What does the W-8BEN really do? What if I do not or forget to submit this?
2) Even after filling up the W-8BEN form, 30% of my dividend payout
are being withheld. What do tax withholding really mean in layman
terms? Will I ever get to recover these money?
3) For the other 70% of the dividend payout, do I still need to
declare this income in my country again as taxable income? (If so,
I'll be tax again!)
4) I spoke with the USA broker and they say that Singapore do not have
a treaty with USA, what does it mean to me with regards to the above?
5) For the affiliate commissions, should they withhold 30% of the
income too? I read somewhere that the withhold applies only when the
business is "conducted" in USA, but since it's an e-commerce with
customers all over the globe, do I still subject to this tax
withholding? And if I do, what does it mean to me when it comes to
declaration of tax in my country (again, double tax?)

While this question seems long, I'm only trying to be specific. Hope
to get some layman answers for these.