可以簡單說說什麼是次按問題and點解佢可以影響股市

次按問題 means sub-prime mortgage problem. Borrowers who have poor credit rating and can only borrow money at higher interest rate are the sub-prime borrowers. However, US economic downturn due to falling housing sector will have cause more people become unemployed and hence the borrowers can not repay their debt. The banks who have bought the derivatives from those sub-prime mortgage loans can not get back the money, thus writing off those assets. The banks will incur huge loss from those investments hence stock market collapsed.