The (1) -- The definite article

The word the is known as the definite article. Its use in English is a little tricky, and perhaps the best way is to approach the matter gradually. Accordingly, I am dividing the articles on the definite article into a number of separate entries that increase in difficulty as they progress numerically. 

Let us begin with three very simple examples that pose no problems, and consider why they are so easy to understand.  We may then use that understanding to build up  our competence.

When I began to explain the use of the definite article in class, a student began to weep silently. Between sobs, the student said he would never be able to master this aspect of English.

I wrote a book that no-one wanted to read. Now, when I see the book sitting in my house, I feel bitter and untalented.

Jennifer has a small dog with a snub nose and fat face. The dog is a Chihuahua.

In each of the examples above, the indefinite article a  in the first sentence becomes the definite article the in the second because the noun has now been identified. First it was a student (we don’t know who), then the student (we don’t know his name, but we know it’s the same person from the previous sentence); a book (we know nothing about the book), then the book (we may not  know its title yet, but we do know that  it is the book just  mentioned);  a dog then the dog (the one referred to in the previous sentence).

Rule: On first introducing a noun, use a; thereafter, use the. 

Reason: The noun has been identified.

Once something has been identified (by you in a previous sentence or by people in general), use the definite article.  This should explain why certain words such as the moon, the earth, the sun use the definite article – they have been identified a long time ago by humans, and there is only one of them.

Naturally,  this approach  only works if we are certain that the noun identified is the one we mean. The moon is not made of cheese (i.e. the large satellite up the in the sky that humans have been looking at since humans existed), but: Ganymede is a moon that orbits Jupiter.