When to omit un/una?

[SPX]

So I'm working my way through the Rosetta Stone Spanish program and it hit upon something that I've never been clear on. When, and why, do you omit un/una?

For instance, some of the examples they give are:

Él es médico.
Ella es mestra.

Why do you omit un/una?

After all, he is A doctor. She is A teacher.

[Luna Azul]

In general, when talking about a profession, we omit the article.

"Él es médico"
"Él es muy buen médico"

But:

"Él es un médico muy bueno"

Mind you, you can say "Él es un médico" and "ella es una maestra" but those are less commonly used.

Frankly, I can't tell you why. I've no idea if there's a rule, maybe someone else will give you a better explanation.

[Rusty]

You omit the indefinite article when designating a person by profession, but both ways are allowed.

If you want to say (or emphasize) that the person is one of several doctors, then you would use the indefinite article. Conversely, the only doctor in a particular place would be designated with the definite article.

[SPX]

Thanks for the responses. Very helpful.

What if you are talking about a student, for instance? Or someone who has a hobby, but doesn't do it professionally?

Speaking of students, is both "estudiante" and "alumno" equally correct?

[Rusty]

You can omit the indefinite article in those cases, too.

Soy estudiante.
Soy alumno.
Both are correct usages, but not entirely interchangeable.
An 'alumno' is a student in a school or a university.
A 'student' describes a person enrolled in a class, but it also describes a person that is studying a topic or field on their own, outside of the classroom. A student of Spanish could be self-taught. An 'alumno' of Spanish is a pupil in a Spanish class.

[SPX]

Ah, thanks.