英语语法：关于冠词的问题
有些名词用定冠词修饰,有些则没有,从意思上我觉得差不多啊,这我觉得搞不懂.就是在类指和泛指搞不清楚
例：
1.Attracting senior citizens,Florida is home to the largest population of elder Americans.
"Attracting senior citizens"这之中问什么不能在senior citizens前加“the”,我觉得在指"senior citizens"这一类人,有何不可?
2.Using illegal drugs can lead to health problems,and it an be a stain on one's life.
"Using illegal drugs "中“illegal drugs ”前加"the"解释为“那类违法的药品”为什么不行?
3.The car goes faster than the bike.为什么这里“car”和“bike”前要加“the”呢?如果把“car”和“bike”变为“cars”和“bikes”那还要不要加“the“吗?前两个例子中“the”和他们都是复数有关吗