Omission of "the"

My Italian girlfriend can speak English, but she is having trouble about knowing when to leave "the" out of sentences. I've told her that in plurals, "the" is often omitted-

e.g. "I like THE rainbow"

or "I like rainbows." When it's plural, "the" is ommited. But it confuses Italian speakers, because they say "I like THE rainbows."

But I've noticed cases of plurals in which "the" is left in the sentence. For example, we can say "I used to work in THE coalmines when I was younger."

So, how do we know in which plural sentences "the" is omitted, and in which plural sentences "the" is left in? I'm an English speaker, but even I find it hard to explain to someone from Italy.

In many languages, it goes

THE DOG. Dog , the generic concept.
THAT DOG. The dog we are talking about.

in English it is

DOG. For centuries dog has been the friend of man. Which dog? Which man? It's not important.
THE DOG. The dog we are talking about.

I've forgotten the grammatical names but you understand, I think...

"For centuries the dog has been the slave of man." I'd have put the "the" in the sentence you gave Simon.

Jim, The word in that sentence was ''friend'' not ''slave''.

Steve,

Yeah, I know: I took the liberty of changing the sentence.

I have found a great site explaining about how use articles in English. You could advise your girlfriend to read the following article:
http://www.rpi.edu/web/writingcenter/esl.html

Ok replace dog with "sunlight".

"For centuries the sunlight has been the slave of man."

Hmmm ... that's nice.

Asian languages have the opposite problem. Someone from Japan or China might say "I like rainbow."

Same difficulties in french, i like rainbows = j'aime les arcs en ciel, the = les wich is all the time used in french. It's difficult in english beacuse we have the impression to speak like Tarzan if you can understand what i mean. Do not think it's unrespectfull, it's just the idea.

At school a Chinese boy wrote "Behind wall have house". It's a literal translation of Chinese but difficult to understand in English.

Okay. Thanks for your help.