Articles are necessary in a noun phrase in order to indicate whether the noun is definite or not. There are some situations when the article can be omitted.
English articles are divided into two categories: indefinite and definite.
Indefinite articles (a/an)
In English we use the indefinite articles "a" and "an" with singular countable nouns in the following situations:
If you mention for first time a thing
Example: I saw a nice car).
When you say what something is
Example: It is a nice car).
When you say what somebody does (like a job)
Example: I am a teacher/ I am an English teacher).
In exclamations with “what”
Example: What a smart boy!).
Some expressions like “I play tennis three times a week”.
Indefinite articles in English language are used before the noun and indicate a specific person or thing that is unfamiliar to the listener or the thing or person hasn’t been mentioned before (example: I saw a nice car). Indefinite articles can indicate a whole class of things or people (example: I play tennis three times a week).
Definite article (the)
English has only one definite article, which is “the”.
In English we use the definite article "the" in the following cases:
When we talk about something we have mentioned before
Example: I saw a beautiful woman driving a car and the car was red. (“car” was mentioned before so my listener knows what car I am talking about)
When there is only one of something (so, the definite article makes the noun unique)
Example: The Earth goes around the Sun. (there is only one Earth and one Sun)
When is clear what we are referring to
Example: He opened the door.
With places like: bank, theater, supermarket, park etc
Example: I am going to the supermarket.
Example: It is the cheapest car.
There are some situations when we don’t use articles in English. Don’t use articles in the following cases:
Speaking in general
Example: Motorcycles are faster than bicycles.
With some nouns that follow one of the words "at/to/from"
Example: She went to school.
Before meals, days, months
Example: I always play tennis on Sunday.
Before the words “next” or “last” followed by days, weeks, etc
Example: See you next week.
Teaching articles for ESL learners is not an easy task. It requires a lot of practice and the mistakes have to be corrected immediately and explained.