Rules: 1) if it is in general there is no article. Example: "all cats like mice". That is ALL cats, always. 2) if it is 'one of this kind of thing' or 'any one of these' then it is a/an. Example: "Can I borrow a pen?" - any pen - you don't mind, you just need A pen. 3) if it is a concrete/specific thing which you can describe in detail, because it is specific then the. Example; "the green pen/s"