Hello!

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?

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

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?

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.

Hello! I can definitely help you understand the usage of definite, indefinite, and zero articles in English.

Definite Article (The):
The definite article "the" is used to refer to a specific noun or a noun that has already been mentioned in the conversation. Here are a few situations where you would use "the":

1. When referring to a specific noun: "I saw the cat on the roof."
In this case, "the cat" refers to a particular cat that both the speaker and the listener are aware of.

2. When referring to a singular noun that is unique: "The sun rises in the east."
There is only one sun, so "the" is used to refer to it.

3. When referring to a group or category as a whole: "I love the beach."
"The beach" here represents the concept of beaches in general.

Indefinite Article (A/An):
The indefinite articles "a" and "an" are used to refer to a non-specific or unspecified noun. Here's how you can use them:

1. When referring to a singular countable noun for the first time: "I bought a book."
"A book" refers to any book in general, not a specific one.

2. When referring to an occupation or nationality: "She is a teacher."
"A teacher" doesn't refer to any specific teacher; it could be any teacher.

3. When stating an amount or quantity: "I saw an elephant at the zoo."
"An elephant" could be any elephant.

Zero Article:
The zero article is used when there is no article before a noun. Here are a few cases in which the zero article is used:

1. When referring to general concepts or ideas: "Cats are adorable animals."
"Cats" here refers to all cats in general.

2. When referring to plural or uncountable nouns: "I love strawberries."
"Strawberries" doesn't refer to any specific strawberries, but to the fruit in general.

3. When referring to places, languages, meals, and certain expressions: "I go to school."
In this case, "school" is referring to the general concept of attending school.

To determine which article to use or when to omit it, it's important to consider the context and the specific rules for each case. Practice and exposure to the English language will help you become more familiar with the usage of articles.