Когда множественное "the" а когда единственное число "a". И еще в названиях фильмов, рек,всяких достопримечательностей, но это уже нудно запоминать.
1) A stand for the environment is taken by people.
2) Swimming in the polluted lake is forbidden by them.
3) Trees are sawn down by people.
4) The beach was closed by them because of the flood.
5) In order to help the environment, local products are bought by people.
Если зАмок - castle
если замОк - lock