You sould eat more food.
Children of your age have to eat more food.
If you want know english better, you should spend more time warking with your dictionary.
They should learn the rules of using the library.
Students should return themselfs(возвращаться)/return books(возвращать книжки) to the library in time. (Вообще-то, "in time" это вовремя, но как переводится "належний час" я не знаю.)
We should work hard if we want have time to watch the concert.
He should not ride a bike in (или "at") the street, where is heavy traffic.
You should not use dictionaries during your examination of tests.