When to include or omit 'a' (un, una)?

Question

I am studying Spanish. In my textbook it says:

Americans have amazing fashion

Los estadounidenses tienen una moda increíble

Why has "una" been included? The sentence is clearly talking about fashion, which is not singular, so why use "una"?

This Question/Answer says you have to use a when referring to persons, but in the example above, it's referring to fashion, which are not people.

Later on the book, it has the following examples:

Tengo dolor de muelas

I’ve got a toothache

No. Tengo un dolor de muelas terrible.

No. I’ve got a raging toothache.

Again, in one "un" is used. In other it is not.

This Question/Answer says you normally use un to refer to quantity. But then why does one tooth ache have a quantity and the other does not?

When do you include or omit 'a' (un, una)? Based on the examples above, it seems very random.

Answer 1

Very interesting question - I'm native but it made me think about the why though.

As a rule of thumb, if you use un/unos/una/unas you need to continue specifying about the noun in question.

Correct:

Tengo dolor de muelas

Tengo un dolor de muelas terrible

Incorrect:

Tengo dolor de muelas terrible

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And the example about moda is a little more complex.

Again, as a rule of thumb you have to use moda with una and complement it.

Correct:

Los estadounidenses tienen una moda contemporánea

Incorrect:

Los estadounidenses tienen moda

The explanation of why you need to specify fashion is because otherwise it wouldn't mean anything - fashion is a noun too generic.

However, you can use this idiom about fashion:

Los estadounidenses van a la moda

Answer 2

The main function of the distinction between the indeterminate article, just as in English, is familiarity:

la casa --it is assumed everyone knows (or can figure out) which house we're talking about.

while:

una casa --the house is not determined, or is just being introduced in the conversation.

(this example has been partly adapted from this.)

The case here is a bit different, and specific to Spanish.

There is this very good article that explains when not to use the indefinite article:

El artículo indefinido no se usa en los casos siguientes:

• 1. Después del verbo ser cuando se habla de nacionalidad, profesión, oficio o religión no modificado (...)
• 3. Después del verbo tener algunos sustantivos comunes se usan sin artículo para indicar un tipo de cosa en general

(3. after the verb tener some nouns are used w/o article to mean a general kind of thing.)

In your examples, you add the article when you individualize the object: un dolor de muelas terrible is no longer a general type of thing, but this specific ache of yours. As a rule of thumb, in the cases with article, the object (dolor de cabeza, moda) is being individualized, while in the case without article you are referring to the abstract concept.

A [not-quite-perfect-but-useful] mnemonic you could use is:

I’ve got a toothache

Since dolor de muelas is a general kind of thing it is translated without the article, but if you specify it:

I've got a toothache. An incredible one.

You are specifiying it, and the rule of when to omit the article no longer applies.

Answer 3

It is not exactly random but instead those are examples in which you could or could not use the article.

It is perfectly correct to say: Los estadounidenses tienen moda increíble

If you do not use "una" you are using a more general meaning (fashion in general) but with "una" you are being more specific and they may have some amazing fashion among some not as amazing. (I have to say this is not the best example)

The other example is similar. The more specific sentence is Tengo un dolor de muelas terrible but the more general sentence does not use the article tengo dolor de muelas in general.

Un/unos/una/unas are called indeterminate article and in most cases is used to express an indeterminate quantity. i.e.

Trajeron unas cervezas. = They brought some beer.

Trajeron una_ cerveza_. = They brought __a_ beer (any beer).

in contrast to:

Trajeron ____ cervezas. = They brought ____ beer. (no quantity defined)

The part about "a" is also a bit tricky since "a" is a preposition and in my opinion prepositions are among the most difficult things for a beginner in Spanish.

For instance, in cases like "Acabamos de vacunar al ^{(a el)} perro" "a" is the preposition that precedes the direct complement and in some way makes more specific the object. (Def 1)

The explanation on all the possible ways of using "a" would be rather long so I will only say be patient while learning and you will get the hang of it, but if you want to read some examples of the uses of "a" check this link http://dle.rae.es/?id=002rZ9U|003Ov93 (definitions 13 to 21 are quite interesting)