How do I use the subjunctive to describe the unspecific?

I have seen the Spanish Subjunctive used to describe things that I am looking for. For example, ¨Busco un libro que sea bueno¨ and ¨Quiero encontrar un médico que especialice en rodillas.¨ Could you give some more examples and clearly explain the rule?

The subjunctive is used to describe or refer to an unspecific person, place, or thing. By unspecific, I mean, for example, if you have ten apples in front of you and someone tells you to choose a sweet one (escoge una manzana que sea dulce), you would use the subjunctive.

It may or may not help to think of it as describing a characteristic of something instead of a specific thing. For example, ¨escoge el cachorro que más le guste (Choose the puppy that you like the most, or that is most pleasing to you).¨ Rather than describing a specific puppy, it is decribing a specific characteristic of a puppy (that it is pleasing to you).

Compare the following:

Busco a mi amigo.	I am looking for my friend. (a specific person)
Busco un amigo que sea amistoso.	I am looking for a friend that is friendly. (unspecific friend, specific characterisitic=friendly)

Also:

Busco mi libro.	I am looking for my book. (I know it exists=no doubt)
Busco un libro que se trate de gramática.	I am looking for a grammar book. (It may not exist = doubt)
No quiero tener nada que ver con alguien que se comporte tan a lo loco.	I dont want anything to do with someone that behaves so crazy. (unspecific person, specific characterisitic="loco")
Te pasas la vida buscando un remedio que solucione todos tus problemas de un plumazo.	You pass your life looking for a cure that would solve all of your problems with the stroke of a pen.
Yo les invito a todos; los que vengan serán bien recibidos, los otros...¡allá, ellos!	I am inviting everyone; those that come will be well received, the others... thats their problem!

Practice:

Given each of the questions below, type in the correct response.  Although capitalization does not count, spelling does!

Please enter your name

1. Un tipo que (acabar) de llegar del pueblo, no se dejaría engañar como tú.

2. Los ejercicios que (nosotros, hacer) después, serán más difíciles, claro.

3. ¿Hay alguien a quien le (interesar) comprar una cámara y un teleobjetivo muy baratos?

4. Claro que cierro los cajones con llave; no puedo fiarme del primero que (llegar) .

 

On a similar note, the subjunctive is also used to describe the nonexistant and unknown. For example:

No como nada que contega preservantes.

I don't eat anything with preservatives.

This falls within the same category as the subjunctive required after negative statement+ que.