he must keep his mind fresh by setting aside some time for exercises.这句话里的by做什么解释,整句话怎么解释最通顺.

he must __(finish)his homework first. he must change his dietchange changes? he must keep his mind fresh by setting aside some time for exercises.这句话里的by做什么解释,整句话怎么解释最通顺. he sit ( )his desk all day ( )his head( )his hands.he is keep in thought. He wanted to keep it s____from his family. He wears dark glasses___(keep)his eyes healthy. He must finish his hone work.- he - his hone work?No,he- -He-finish his hone work. 请从语法角度加以说明~（1）39.the medicine ________cool,clean and dry.A.must keep B.must be kept C.must be carried D.must be in49.________ his stamps he has collected are from Canada .A.two hundreds B.hundred of C.two hundred of D.two hund he couldn't earn enough to keep himself and his family .keep为什么不用动名词 He must do his homework.改成一般疑问句 He must finish his homework(该为一般疑问句) He must do his homework,的谓语动词是什么? He must have finished his homework,_________________(是吗?） He must look after his sister (改为一般疑问句 ) He thinks it must be his friend's house.是什么意思? Judging from his a________,he must be from the USA. 找出错误并改正：He must finds his English teacher. he must wash his face quickly否定句