1. Should my grandpa be stay in the hospital?
我外公需要住院吗?
还能这样说：
Does my grandpa have to be in hospital?
Does my grandpa need to be hospitalized?
应用解析：
stay ahead (of) 领先，(走在……的)前面;
stay at 暂住在…… 投宿在……
stay away (from) 不在，外出;走开，别接近;
stay behind 留下来不走;留在后面
2. I am checking into the hospital tomorrow.
我明天要去住院。
还能这样说：
I need to be hospitalized tomorrow.
Tomorrow I need to go into hospital.
应用解析：
check at 对……发火;
check in 签到;报到;办理住宿手续;归还所借书籍;
check (up) on 调查，核对，检查;
check with 与……相符合;与……协商
3. You must be hospitalized at once.
你必须立即住院。
还能这样说：
You should go to hospital without delay.
You had o stay in the hospital immediately.
应用解析：
for this once = just for once 就这一次;破例一回;
if once = when once 一旦;
more than once 不止一次，多次;
not once 一次也不;
once again 再一次
4. Do I have to stay in the hospital for long?
我必须住院很久吗?
还能这样说：
Do I must have a long stay in hospital?
Do I have to stay in hospital for a long time?
应用解析：
stay the course 持续到底，坚持到底;
stay by 待在……的旁边，留在……旁边;
stay out of 不参与;不插手;置身于……之外