Twobit

You asked an excellent question.  This is something I always wondered about and it does not seem to be taught in schools.  The text in blue comes from A New Reference Grammar of Modern Spanish fourth edition by John Butt and Carmen Benjamin.

Section 3.3.8 Omission of nu/una after other verbs

Spanish does not use un/una after a number of verbs such as tener ‘to have’, comprar ‘to buy’ sacer ‘to take/draw out’ (with cinema tickets, etc. ‘to buy’ or ‘to book’) , buscar ‘to look for’, llevar ‘to wear’ , when their direct object is a noun referring to things of which one would normally have or carry only one: umbrella, pen, spoon, nanny, valet, cook, hat.

Pepe ya tiena secretaria – ‘Pepe’s got a secretary now’

¿Tenías idea de lo que serías capaz de hacer? Did you have any idea of what you’d be able to do?

Ya he scado entrada – I’ve already got a ticket.

…..

(i)                  The indefinite article is used if the object has special characteristics: ‘llevaba una falda blanca’ –‘She wear a white skirt’

(ii)                Use of un/una with unqualified nouns may hint at some suppressed comment: tiene un coche/una casa – ‘you should se his car/house/

You also do not use the article with things like professions, occupations, social status, sex, unless there is a qualifier.

For example :

Soy piloto – I am a pilot

But

Soy un piloto bueno – I am a good pilot.

Me pregunto si se puede decir "si serías tan amable".

For me the interesting thing is that is not conditional, as in English. In the indicative, to me it sounds like a challenge. Show me, be nice!

I wonder if you can say "if you would be so kind."