The definitive article?

A quick recap for those who don't know, indefinite articles in English are used when you are referring to an object in general (not a particular one). Here are some example of the indefinite articles a and an in use

I'm reading a book
I'm eating an apple

In English the word the is a definite article, use it when you are referring to something specific (sometimes something you have previously mentioned).

The book I'm reading is very interesting

Foreign learners often get confused over which ones to use. Or maybe in their native languages definite and indefinite articles aren't used and they simply omit them in speech. This can have the effect of making them sound rather abrupt/surly in conversation. To complicate matters further, there are occasions when it is quite correct to drop the articles.

I like reading books (no article needed here)

When talking about abstract concepts such as feelings the article is often dropped.

Love is a many-splendoured thing (here the a is the indefinite article attached to the word thing)
Hate is a destructive emotion
I can feel the love in the room (here we are talking about specific warm feelings in a specific room)

Sometimes the word "the" is emphasised slightly.

I met a great guy at the library, I think he may be the one.

(this implies that this person is the one that you want to spend the rest of your life with)

Mr Right turns into Mr Angry when he is having problems at work and confides in his girlfriend "My boss is driving me crazy, sometimes I want to hit him". Apart from advising him to go on an anger management course here are a couple of pieces of advice she may offer.

Violence is never the answer
Violence is never an answer 

The difference between these two is quite subtle. The first sort of implies that out of all the possible answers violence is one of the possible answers (albeit inadvisable). The second answer suggests that violence shouldn't even be included in the list of possible options.

You will often see definite articles used in sayings and idioms, e.g.

The pen is mightier than the sword

Consider the following (grammatically correct) alternatives

A pen is mightier than a sword
Pens are mightier than swords

The original version is best, we're talking metaphorically here. The pen is a symbol of language and rhetoric, the sword is a symbol of weapons and violence in general

Hopefully the examples I have given have helped to clarify the matter. Ultimately, practice makes perfect.