The definite article is used to indicate the specific noun or nouns that you are talking about.
|The man with red hair is my father.
(There are several men over there – the one who has red hair is my father.)
|Here is the book I bought.
(I bought a book and now I am showing this specific book to you.)
|I like the south of France.
(The south is a particular region of France, and it is the one that I like.)
|The red skirt is prettier than the blue one.
(You are looking at two specific skirts and you prefer the red one.)
|Where are the children?
(You are asking about a particular group of children.)
The definite article is not used in English when referring to to abstract ideas, a noun, or a group of nouns in general.
|I like science fiction.|
|France is a beautiful country.|
|Red skirts are sexy.|
|Children express their creativity.|
|Diplomacy is important.|
Note that a distinction is being made between the red skirt (the one you are wearing, for example) and red skirts (all red skirts, red skirts in general). This same distinction is being made with the children (a particular group of children) and children (children in general, all children).
If you can’t decide whether to use the definite article in English, think about the meaning of the noun. Is it a specific item (that you see, own, or are wearing) or is it an unspecified group, a general idea? If it is general, do not use the definite article.
When referring to a person with a title, do not use an article – the title is used instead.
|President Obama is in Italy.|