Necesito un amigo que sea famoso. (I need a friend who is famous.)
Does this friend exist? Maybe, but for now he is in my mind. I have an idea of what I need or I'm looking for. However, THAT IS AN IDEA. This is why we need the subjunctive to describe the friend I'm looking for.
¿Conoces a alguien que tenga dos perros? (Do you know anybody who has two dogs?)
In this question, the person asking has no clue if you know a person who has two dogs. Ok, there is no adjectve here, but the description works like an adjective.
The “subjunctive in adjective clauses” simply means that the clause with subjunctive contains an adjective (description) of a noun (person, thing, place, etc.) that may or may not exist. The phrase in this case is formed with two sentences,
sentence 1: indicative (noun) + que + sentence 2: subjunctive (adjective that modifies the noun from sentence 1)
Let's see more examples.
El libro trata de la vida en América Central. (The book is about the life in Central America.)
There is no subjunctive here, because we are talking about “THE BOOK,” which means that the book is known to either the speaker or the listener of this sentence.
Busco que se trata de la vida en América Central. (I'm looking for that is about life in Central America)
There is certainty here. This is why there is no need for subjunctive. The next sentence is similar to the previous one, but it has subjunctive,
Busco un libro que se trate de la vida en América Central. (I'm looking for a book that is about the life in Central America)
Usually the key is the indefinite article (un, una, unos, unas), and indefinite words like something, somebody or someone (alguien, algún, alguna, alguno, algunos, algunas, algo).
Some of the verbs we use often with these structures are: buscar (to look for), necesitar (to need), conocer (to know), querer (to want, to love).
Remember that you need to have two sentences (two conjugated verbs).
Consider this example:
Necesito un trabajo bueno. (I need a good job.)
There is no subjunctive. There is only one verb.
The next example contains subjunctive:
Necesito que sea
(I need a job that is good.)
Again, it has subjunctive because there is uncertainty.
Here are a few more examples:
Necesito el apartamento que está en el centro de la ciudad. (I need the apartment that is in downtown.)
Necesito un apartamento que esté en el centro de la ciudad. (I need an apartment that is in downtown.)
Ella busca al chico que es guapo e inteligente. (She is looking for a boy who is handsome and intelligent.)
Ella busca a un chico que sea guapo e inteligente. (She is looking for a boy who is handsome and intelligent.)
Conozco a la chica que se llama Lupita. (I know the girl whose name is Lupita.)
Conozco a una chica que se llama Lupita. (I know a girl whose name is Lupita.)
No conozco a una chica que se llame Lupita. ( I don't know any girls whose...)
El libro es caro. (The book is expensive.) There is no subjunctive here.
Necesito el libro que es caro. (I need the book that is expensive.)
Necesito un libro que sea caro. (I need a book that is expensive.)
¿Conocieron ustedes que se llama Pancho? (Did you meet whose name is Pancho?)
¿Conocieron ustedes a un chico que se llama Pancho? (Did you meet a boy or any boys whose...?)
I hope this helps.