Articles are used to connect nouns to a sentence and are very important in the English language. The secret to choosing articles correctly is easy, there are only two things you need to remember: First, nouns can be countable (things that are easily counted) or they can be non-countable (things that are not easily counted.) They can be singular (one of something) or plural (more than one of something.) Second, you need to understand what kind of noun you are using in your sentence. So, let's look at the three basic kinds of nouns.

Generic Nouns

Generic nouns are used as symbols. These nouns are not real, they are not specific, they are not concrete or tangible items or things. They are rather a symbol of something that is commonly know.

A banana is yellow. An apple is red.
(A or an is used with singular countable generic nouns.)

Bananas and apples are good for you.
(No article is used with plural countable generic nouns.)

All fruit is healthy.
(No article is used with non-countable generic nouns.)

Indefinite Nouns

An indefinite noun is a real thing, but it is not specifically identified and is not important to do so.

I ate a banana and an apple this morning for breakfast.
(A or an is used with singular countable indefinite nouns.)

I ate two bananas and a few pieces of apple for lunch.
(Other words like: a couple, a few, several, a lot of, some, many, two, three, etc. can be used to replace articles with plural countable indefinite nouns.)

I ate some fruit this afternoon for a snack.
(Other words like: a lot of, some and much can be used to replace articles with non-countable indefinite nouns.)

Definite Nouns

Definite nouns are real things specifically known by both the speaker and the listener.

Thank you for the banana and the apple.
(The is used with singular definite nouns.)

Thank you for the bananas and the apples.
(The is used with plural definite nouns.)

Thank you for the basket of fruit.
(The is used with non-countable definite nouns.)