Articles, a tricky problem in English. But there is a simple system. 1) Ask yourself - is this in general? For example "cats like mice". No articles; because ALL cats like ALL mice. 2) Ask yourself - are we talking about a specific thing, that I can see/touch (for example) and can give lots of information about? - in this case 'the'. 3) Am I talking about "one of this type of thing?" - then 'a/an'