Preparing for the 'oposiciones': because/as/since/for

As, since, for and because are the conjunctions most often used to introduce adverbial clauses of reason (subordinadas causales):

I'm late because I missed the bus (Llego tarde porque perdí el autobús)
As it was raining, I stayed at home the whole day (Como estaba lloviendo, me quedé en casa todo el día)
Since you insist, I'll tell you the truth (Puesto que insistes, te diré la verdad)
He decided to stop, for he was a bit tired (Decidió parar, pues estaba un poco cansado)

Because is the most frequent, and the only one used to answer why-questions directly:

"Why didn't you come yesterday?" "Because I wasn't feeling well" ("¿Por qué no viniste ayer?" "Porque no me encontraba bien")

and for, more formal in style, is the least used of the four. 

Clauses introduced by because, as or since can be placed before or after the main clause, though those introduced by because are usually placed after the main clause, and those with as or since, before:
I won't tell Charles, because I don't trust him (No se lo diré a Charles, porque no me fío de él)
As it is snowing, I prefer to stay at home (Como está nevando, prefiero quedarme en casa)
Since we live near the sea, we often go sailing (Como vivimos cerca del mar, hacemos vela a menudo.

However, those introduced by for always go after the main clause: 

I can't go out with you today, for I have to study for my English exam next week (No puedo salir hoy contigo, pues tengo que estudiar para mi examen de inglés de la semana que viene)

Finally, it should be borne in mind that because is the only one of these four conjunctions which can be preceded by and:

No, I won't go with you to the cinema, because I'm very tired and because I hate horror films (No, no iré al cine contigo, porque estoy muy cansada y porque odio las películas de terror).

PRACTICE

Join the following clauses by means of the conjunction given in brackets. Then translate into Spanish:

1.- I can't buy that dress. I have no money left (because)
2.- You can buy it now. The circumstances have changed (as)
3.- The money hasn't been paid into my account yet. I can't pay you (for)
4.- I didn't dare to contradict him. He's the boss (since)
5.- His mobile wasn't working. He couldn't call his wife  (because)
6.- Everybody wanted to dance with her. She was the prettiest girl in the party (as)
7.- We'd better change the subject. You're not going to convince me (since)
8.- He got caught in a traffic jam. He missed his flight (because)
9.- The information was top secret. The Minister could not divulge the Cabinet's deliberations (for)
10.- The computer programme crashed. She lost all her data (because).

KEY

1.- I can't buy that dress because I have no money left (No puedo comprar ese vestido porque no me queda dinero).
2.- As the circumstances have changed, you can buy it now (Como las circunstancias han cambiado, puedes comprarlo ahora).
3.- I can't pay you, for the money hasn't been paid into my account yet (No puedo pagarte, pues no me han ingresado el dinero todavía).
4.- Since he's the boss, I didn't dare to contradict him (Como es el jefe, no me atreví a contradecirle).
5.- He couldn't speak with his wife because her mobile was out of service (No pudo hablar con su mujer porque el móvil de ella no estaba operativo).
6.- As she was the prettiest girl in the party, everybody wanted to dance with her (Como era la chica más guapa de la fiesta, todos querían bailar con ella).
7.- Since you're not going to convince me, we'd better change the subject (Puesto que no me vas a convencer, mejor será que cambiemos de tema).
8.- He missed his flight because he got caught in a traffic jam (Perdió su vuelo, porque se vio cogido en un atasco).
9.- The Minister could not divulge the Cabinet's deliberations, for the information was top secret (El Ministro no pudo divulgar las deliberaciones del Consejo, pues la información era alto secreto).
10.- She lost all her data because the computer programme crashed (Perdió todos sus datos porque el programa del ordenador se bloqueó).

For other conjunctions used to introduce adverbial clauses of reason, see my Gramática Inglesa, 9ª ed., p. 526, and for that, used as an adverbial subordinator of reason, p. 604, note 3.