It is necessary for us to study math.中为什么要写us而不写we呢

it is necessary for you to work It is necessary for sb to It is necessary for you to take an X-ray. It is necessary for me to have some help 翻译 It is necessary for us not to eat fast It is necessary for us_______ our classroom clean. It is necessary for people to make a call It is necessary for you to help each other.翻译 Is it necessary for employees to keep single in companies Do you thing it is necessary for sencer to study__? it is necessary for me to take a taxi中用动词来代替it is necessary for 英语necessary的问题It is necessary for us to learn.可不可以改成We are necessary to learn 是it's necessary for us to sleep for 8 hours 还是is' necessary for us to sleep 8 hours? It is necessary for us to help each other(同义） _________ __________ ________ _________is necessary for us it is necessary to ask for time to think it over It is necessary for you to take measures to work it out.翻译 英翻中：Modern medicine is developing so quickly that it is necessary for doctors to keeplearning I think it is good for me.I think it necessary to…… it后什么时候有is