Why the balance of payments accounts must balance?

The Balance of Payments is the sum of the Current Account and the Capital Account. The Balance of Payments Identity states that:

Current Account + Capital Account = Change in Official Reserve Account

http://en.wikipedia.org/wiki/Balance_of_payments (入面仲有好多link 去related issues, 真係好有用)

美國NY Fed 既 viewpoint
http://www.newyorkfed.org/aboutthefed/fedpoint/fed40.html


好多國家既Change in Official Reserve Account 都好少, 或者少到可以當無. So, normally
Current Account + Capital Account = 0
又或者
Current Account = - Capital Account (既係個數字balance)

點解 must balance?
The basic principle behind the identity is that a country can only consume more than it produces (a current account deficit) if it borrows from abroad (a capital account surplus, where 'borrowing' includes all forms of investment from abroad). 好簡單咁講, 你要洗錢 (consume), 但又多過你earn 既, 個Current account 會係 負. 咁你就要借番你爭既錢去比, 所以一定係balance的.
倒過來, 一個國家賺多過洗, 就有錢投資海外. (其實兩個account入面包括好多野, 呢度只係深入淺出)

Balance means Current Account + Capital Account = 0.

Why despite this there can still be problems?
通常都會睇美國做例, 美國 current Account 負左(deficit)好多年 (主要因為入口多過出口, 如中國).
邊度來錢? 點balance番? 借錢, 美國isuue 既國家債券 係全球最多既(2006 年有US$ 9,000 billions!), 好多美國官員同有d經濟學家都怕會有problems, 咁多債幾時先還到? 至於還唔還到/太多debt係米真係問題, 其實經濟學家有兩睇的, 可以話未有結論 (因為econ有好多派). 只可以話 '可能' 對 interest rates/exchange rate/foreign ownership of US 有影響. 可睇下以下既discussion:
http://en.wikipedia.org/wiki/United_States_public_debt
http://en.wikipedia.org/wiki/United_States_public_debt#Paying_the_debt_and_arguments_against_doing_so

希望答到你想要的, 同埋d 資料對你學習有用啦 ^^