Hola Bill

There isn't a simple rule for using or not the definite article in Spanish -sometimes we have to use it, depending on which verb we use, for example, with gustar we always use it: "Me gustan las películas clásicas"; some other times we could omit it if we want, when we talk about things in general with no intention to specify: "Ella come manzanas" (general) or "Ella come las manzanas" (this is more specific, probably referring to some specific apples in front of you, or making a distinction between apples and another fruit).

I am not sure which sentence you are referring to from the text, but for example, we have:

"Prefiero las películas clásicas" 

We opted here to include the article to make this specific; this way we specify the object as opposed to other types of films. Also, there is a tendency to use the article with this type of verbs showing preference. The same way we include the article in this other sentence from the text, where we are using "odiar" in which case it always takes the article:

"Y odia los perros" 

It'd be incorrect to omit it and say "Y odia perros".

Here are some specific lessons about the use of the definite article, have a look.

Presence of the definite articles el/la/los/las

Using definite articles el, la, los, las with bodyparts and clothing 

Using the definite article or not with titles

Saludos

Inma

Hola Bill

If you are simply referring to when to use el and when to use la, then the answer is much simpler : ))

Use el in front of masculine nouns 

Use la in front of feminine  nouns 

We have a gender for every noun. The general rule is that if the noun ends in -o it is masculine and if it ends in -a it is feminine:  el niño (the boy), la niña (the girl).

But this is a very generic rule and there are lots of exceptions, plus there are nouns that end in a consonant too.

Here are two links where you will find an explanation and all lessons related to "gender masculine or feminine".

Género femenino

Género masculino

Saludos

Inma