I do teach lessons on this. :) LGF92 has given some really good advice, but some very, very basic advice I give is: if you're not sure, use an article anyway! A wrong article is a much smaller mistake than a missing article.

No article when the statement is general or true for all nouns of a similar description: Tables are big. Pens are useful. Use "the" when referring to a specific object: The table over there is big. / The table is big. This pen is useful. / The pen is useful. i.e. you are referring to ONE object, a specific one be it abstract or concrete; "the situation in Libya", "the economic crisis" as opposed to "a situation in Libya", "an economic crisis". Use "a" when being generic, referring to the instance of something, or nothing in particular. A table can be big. / A table would look nice in our garden. A pen would be useful. / A pen is always handy to have around. There is a situation in Libya. BUT The situation in Libya is severe. (if you said "a situation in Libya is severe" it would mean that there were multiple situations in Libya, of which only one was severe). An economic crisis hit the world in 1993. (if you said "the economic crisis" here you would either be referring to one you had already mentioned, or the only economic crisis ever) So in summary; "the" refers to one specific item or instance, whereas "a" is more generic. You will get the hang of it with practice.

here is good site to help you http://www.english-at-home.com/grammar/articles/

You can tell. If it sounds ridiculous, put an article. For example, which sounds ridiculous and which sounds better? She bought apple or She bought an apple?