Answer to Question 1
Hit by the decrease in housing prices and the collapse in stock prices, and worried that this might be the beginning of another Great Depression, people sharply cut back consumption. Worried about sales and uncertain about the future, firms sharply cut back investment.. Decreases in consumption and investment led to decrease in demand, which in turn, led to decrease in output.
Answer to Question 2
Many of the mortgage loans that had been given out during earlier expansion were of poor quality. Many of the borrowers had taken too large a loan and were increasingly unable to make mortgage payments. mortgage backed securities were so complex that their value was nearly impossible to assess. Not knowing the quality of the assets that other banks had on their balance sheet, banks became very reluctant to lend to each other for fear that the bank to which they lent might not be able to repay. The credit market froze up. Unable to borrow, and with assets if uncertain value, many banks found them in trouble. The bankruptcy of Lehman Brothers put other banks at risk of going bankrupt as well. The whole financial system was in trouble.
