What are articles? I'm not talking about the things you read in a newspaper or a magazine, I'm talking about those little words that are attached to every noun you use: "a/an", and "the". Articles serve the purpose of distinguish a general noun (one of possibly many) from a specific noun. They tell the difference between "a ball" and "the ball".
In English we take articles for granted. It might come as a surprise, in fact, to learn that there are languages (such as Russian, or Polish) that don't use articles at all — they determine specificity based on other contextual cues.
Fortunately for me this year, I won't need those contextual cues, because Italian has plenty of articles to tell me even more about a noun than I know in English! Italian articles not only describe generality or specificity, but also indicate gender and plurality.
The Italian articles are as follows:
un / uno
il / lo
i / gli
That looks like a lot, compared to what we're used to in English, but it's really not so hard, and it's actually pretty useful once you get used to it.
Two masculine articles?
At first, the fact that there are two different articles in the masculine form for each case is really confusing. In fact, I spent several weeks studying nouns and learning articles, fighting with memorization. But as with anything, the more you practice it, the more it begins to make sense.
At one point, I was simply listing vocabulary, testing my own memory, and I couldn't remember which article goes with albergo (hotel), so I tried saying the word with both of them. When I would say il albergo, my ears didn't like it, but when I would say l'albergo, it just kind of "felt right".
This triggered my curiosity, and when I decided that when I got home I would find out the logic behind the difference. I made a list of masculine nouns and a remarkable pattern immediately developed: I never saw lo written anywhere! What I saw was il and l'.
What that means is this: just as in english the choice of "a" and "an" depends on whether the next word begins on a vowel, the choice of masculine article in Italian also changes for words beginning on vowels. Mystery solved!
Well, almost. There's one other condition to consider: when a masculine noun begins with a consonant cluster, the article lo or uno is used, to ease pronunciation. So "the mirror" is lo specchio, but "my mirror" reverts back to il mio specchio because the article no longer precedes a cluster. That's not so hard, is it?
In a nutshell
So if you have the flowers, they are i fiori. But when I take one, it's un fiore. And when I give it to my friend, it's il fiore.
However, if I am walking in the trees, they are gli alberi. If I see one I like, it's un albero. And if I cut the tree down, it's l'albero.