You're omitting some key information there, and your answer is very misleading.
Gli is a plural and
l' is a singular. This link has a pretty comprehensive explanation: http://italian.about.com/od/grammar/a/italian-definite-article-forms.htm
Singular Masculine: lo is for masculine singular nouns where the noun starts with "s+consonant" or a "z" il is for all other masculine singular nouns starting with a consonant. l' is used for masculine singular nouns starting with a vowel
Singular Feminine: la is for feminine singular nouns beginning with any consonant l' is for feminine singular nouns beginning with a vowel
Plural Masculine: gli is for masculine plural nouns where the noun starts with "s+consonant" or a "z". It is also used before masculine nouns starting with a vowel. i is for all other masculine plural nouns starting with a consonant
Plural Feminine: le is for any feminine plural noun
In cases like this, Italian uses the definite article differently than English does. In English, we omit "the" for the general case and include it to specify one instance. In Italian, it's the other way around. They use "the" for the general case and omit it to specify one instance.
It doesn't quite mean the same thing.
In English, when we say "You drink the water" we are talking about a specific case. It's particular water you are drinking. It's the water that someone just gave you. It's the water you've been carrying around with you. When we say "You drink water" we're making a more general statement about your habits.
In Italian it's the other way around. For a specific case it's "Bevi acqua" and for general habits it's "Bevi l'acqua".
The way it was explained to me, Italian uses the definite article the opposite way English does in sentences like this.
In English, when we're talking about a specific instance, we use "the"--
- I drink the water = I drink specific water in a specific situation.
We don't use "the" when we're talking about general tendencies and habits--
- I drink water = I am in the habit of drinking water
In Italian, so I'm told, they do the opposite. They use "the" for general tendencies and habits and leave out "the" when talking about a specific instance--
- I drink the water = Bevo acqua
- I drink water = Bevo l'acqua
There are a small handful of exceptions, but Italian is fairly regular.
For most nouns, if it ends in
-o then it's masculine and singular. To make it plural, change the
il ragazzo = the boy
i ragazzi = the boys
If it ends in
-a then it's feminine and singular. To make it plural, change the
la ragazza = the girl
le ragazze = the girls
Italian handles the definite article a little differently than English does. Omitting the definite article means it's a specific case and using the definite article means it's a general statement.
Bevo acqua = I drink the water (specific)
Bevo l'acqua = I drink water (general)
So, if I understand that correctly, I would say "bevo acqua" to say "I typically drink water," and I would use "Bevo l'acqua" if someone's asking about me drinking from a specific body of water, such as, "Bevo l'acqua é un bitollo." Correct me if I'm wrong, I tried to say, "I drink the water from the bottle."
Other way around.
"Bevo acqua" = I am drinking the water that is in front of me.
"Bevo l'acqua" = I typically drink water (not necessarily at the moment).
"I drink the water from the bottle" would be more like "Bevo acqua dalla bottiglia."