1、You shouldn't go to the crowded place.（改为同义句）
You should （stay away from） the crowded place.
2、Michael has to stay at home to look after his little sister.(改为否定句）
Michael （doesn't have to） stay at home to look after his little sister.
3、We should 【keep our rooms clean】 to prevent the flu.（对括号部分提问）
（What） should we （do） to prevent the flu?
4、Must we finish our homework before nine o'clock in the evening?(做否定回答）
（No）,you （needn't）.