“What shouldⅠdo？”“IfⅠwere you,Ⅰwould write a letter.” If he really tried to learn quickly,he could catch up with his classmates. If you don't take more care,you'll have an accident. Oh,Ⅰ'm sorry. Ⅰwould have knocked on the door ifⅠ'd known you were in here. Had it not been for your cooperation,we would not have reached our destination. ⅠwishⅠhadn't been busy yesterday;Ⅰcould have helped you with your work. Don't worry about my job interview tomorrow. IfⅠshould fail,Ⅰwill try again. IfⅠhave got up five minutes earlier this morning,Ⅰwould be at the station now. Without the greenhouse effect,the climate on the earth would be much colder.