Those people who throw away rubbish don't understand that thus they are slowly killing out planet. They pollute the rivers and air and even don't feel sorry. Air pollution is very dangerous because it can kill everybody. But people continue destroying our planet. They harm our forests, cutting it off for their needs. People must stop, they must think how they can solve this problem. For example we can recycle our rubbish, thus we will help our nature and we won't destroy the enviroment. we must protect our planet for ourselves and our future! People, please, don't close your eyes on environmental problems.
How long have you known mr Smith? - I have known him for 3 years
Время Present Perfect (Он знал м-ра Смита в и до сих пор знает; не знаю, как "красиво" объяснить :) )