Q:

Use of the indefinite article in Italian vs. English

In English, we would say:

I am a boy.
We are boys.

using the indefinite article in the singular case.
Is this the case with Italian as well? In other words, would you translate the above sentences as follows?

Io sono un ragazzo.
Noi siamo ragazzi.

In addition, as a complete beginner student of the Italian language, can I generally assume English and Italian articles to function the same way?

A:

The rules are significantly different, which is why Italians speaking in English often get their articles wrong. Good news for you: the other way round is much easier.
As a thumb rule, if you have an indefinite article in English, keep it in Italian; if you have a definite article, keep it in Italian; if you have no article in English, put a definite article in Italian. This applies to both singulars and plurals.

Democracy is a delusion. La democrazia è un'illusione.
Boys don't cry. I ragazzi non piangono.
The neighbour screamed: blood was all over the sheets. Il vicino urlò: il sangue era sparso sulle lenzuola.
A clown was smiling to children passing by. Un pagliaccio stava sorridendo ai bambini che passavano.

There are many exceptions and one is precisely what you picked for your example: a sentence with no article in English that would use the indefinite article in the singular form. I'll use another example (yours has a defect I'm going to explain later):

There was a flower on the bed. C'era un fiore sul letto.
There were fresh flowers on the bed. C'erano [dei/alcuni] fiori freschi sul letto.
There were some fresh flowers on the bed. C'erano [dei/alcuni] fiori freschi sul letto.

All three translations are OK (but the last one sounds worse) and convey the same meaning:

C'erano fiori freschi sul letto.
C'erano dei fiori freschi sul letto.
C'erano alcuni fiori freschi sul letto.

That dei does not mean of the, it's called a partitive article and it's like the plural of un/uno, sort of like some.
(The problem with your example is that it's OK to say Noi siamo ragazzi or Noi siamo dei ragazzi but there is a slight difference in meaning, in these sentences. With no article, it sounds as if you are underlining the age, as opposed to Noi siamo adulti; notice that adulti is an adjective which makes ragazzi sound like an adjective too, even if it is not. This is why I wouldn't pick it as an example, you could say Quando ero ragazzo with no article for the same reason and it has nothing to do with the plural and article rules, it's just ragazzo being used as a kind of adjective.)

A:

Your translations are correct. In particular, in Italian the indefinite article can never have plural meaning (although it can have collective meaning), as in English. Sometimes you can use the partitive article ("dei"), but it is not very common.
I have no systematic knowledge of when you use the articles in the two languages, but I would say that regarding the usage of the indefinite article they are quite similar. Incidentally the sentence I have just written provides a counterexample:

I have no systematic knowledge of ...

becomes

Non ho una conoscenza sistematica di ...

On the other hand

I go running once a week

becomes

Vado a correre una volta alla settimana

Here the indefinite article becomes preposition + definite article.
However, differently from the first example which is applicable to more or less every negative sentence, the second example seems to me more specific, tied to a particular idiomatic construction in English.
So, I would say that the usage of indefinite articles is mostly similar, except in negative sentences like mine above where the English particle "no" stands in the position of an indefinite article. Since Italian has no way to produce a negation just by changing an article or pronoun (you always have to negate the verb itself with "non"), the English "no" particle has to be traslated with an entirely negative sentence; so the "no" particle itself can become an ordinary indefinite article (it could also become "alcuno" or "nessuno", which are more properly translations for "any" and would stress even more the negation in Italian). Then there are many other situations where the usage of the definite or indefinite article (or no article at all) depends on a specific idiom.
Let me just mention an occurrence which I find funny: the three expressions "il giovedì", "un giovedì", "giovedì" roughly translate to English as "(usually) every Thursday", "one of the next Thursdays (when we have time, probably)" and "next Thursday".
For the definite article the issue is more involved. Again, I have no systematic knowledge, but my general feeling is that English tends to use it a bit less. For example it does not use it in front of uncountable nouns and plural nouns with a specification:

Little children are pretty

which in Italian is

I bambini piccoli sono carini

However the matter is much more complicated and is not the subject of your question.

A:

Your translation is ok. In Italian we may also use in the plural the "partitivo": Noi siamo dei ragazzi. 
The main difference I remember in English vs. Italian articles is that we don't have the distinction bewteen countable and uncountable names: Steel is strong becomes L' acciaio è robusto.

