When do we have to use the article 'the' and when we don't have to use it?

Read the paragraph. in fact, teenagers contribute a great deal to their community. this is another reason why they must be taken into account when public policy is debated. over eighty percent of teens spend time volunteering at their schools and other organizations. this is an example of statistical analysis layering point of view outlining

We shall not always plant while others reap

Which is the primary difference between the story of theseus and hercules