Hello!

Question

I know articles are really complicated in English language but what is the easiest way to explain the usage of definite, indefinite and zero article?

Answer 1

These have very good explanations and examples, but I don't know if any of it can be called easy!
http://grammar.ccc.commnet.edu/grammar/determiners/determiners.htm

and

http://grammar.ccc.commnet.edu/grammar/determiners/determiners.htm#articles

Here are some more websites/pages that do a good job explaining all this:

http://www.englishclub.com/grammar/adjectives-determiners-the-a-an.htm

http://wwwnew.towson.edu/ows/determiners.htm

Answer 2

One question: The rule says the we use the definite article with ordinal numbers.I have just seen the example saying "a second letter". Is there some rule when to use it and when not to?

Answer 3

Here is another with some good explanations and examples:
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You would use a, an, or the (whichever fits best) with ordinals when you are indicating a particular place or position in the sequence:

Today is the tenth anniversary of our wedding.

I have a second laptop computer that I use when I travel.

In addition to these articles, other adjectives and possessive forms are often used:

Next week, we'll celebrate her eleventh birthday.

They will spend their fiftieth anniversary on a Caribbean cruise.