In both Spanish and English, conjunctions are words or phrases that connect other words and clauses in sentences. Certain conjunctions commonly introduce adverbial clauses, which describe how, why, when, and where an action takes place.

Main clause + Conjunction + Subordinate clause

Llamaré a la secretaria a fin de que escriba la carta.
I will call the secretary so that she writes the letter.

Cerremos la puerta no sea que regrese.
Let´s close the door lest he comes back.

Adverbial conjunctions, such as those below, are ALWAYS followed by the subjunctive because they always indicate a pending/hypothetical action or state.

a fin de que - so that
a condición de que - on the condition
a menos que - unless
a no ser que - lest, unless
antes (de) que - before
con tal (de) que - provided that
en caso de que - in case
no sea que - lest, so that ... not
no fuera que - lest, so that ... not
para que - so that
sin que - without

Note: Where de is given in parentheses above, it is optional; that is, the conjunction "before" may be expressed as antes que or antes de que.

No voy a menos que venga ella.
I'm not going unless she comes.

Salgo a la una con tal que se termine todo.
I leave at 1:00 provided everything is done.

Trabaja mucho para que vivan bien.
She works hard so they (can) live well.

No hago nada sin que lo sepan.
I don't do anything without their knowing it.

Normally a preposition is used when no change of subject is involved; it is followed by an infinitive, not the subjunctive or indicative.

Mi hijo vive para tocar la guitarra.
My son lives to play the guitar.

Ganaré antes de salir.
I'll win before I leave [before leaving.]