The subjuntivo is primarily used to express will, influence, emotion, doubt, or denial:

• Will and influence
  • Quiero que tú vayas al médico.
  • I want you to go to the doctor.
  • El doctor recomienda que beba menos alcohol.
  • The doctor recommends that I drink less alcohol.
• Emotion
  • Juan espera que su hija deje de fumar.
  • Juan hopes that his daughter quits smoking.
  • ¡Es terrible que necesites un corazón nuevo!
  • It's terrible that you need a new heart!
• Impersonal expressions (verbs that express an opinion with ser + adjective)
  • Es importante que laves los platos.
  • It's important that you clean the dishes.
  • Es malo que ella muriera ayer.
  • It's bad that she died yesterday.
• Requests
  • Insisto en que hagas tus deberes.
  • I insist that you do your homework.
• Doubt and denial
  • No creo que él esté en casa.
  • I don't believe that he is at home.
  • Dudamos que el avión sea seguro.
  • We doubt that the plane is safe.
• Ojalá
  • Ojalá que haya tiempo para hacer deporte.
  • Hopefully I have time to do sport.

To remember these six cases in which we use the subjuntivo, you should take a look at the first letters of the 6 cases. Combined they form the word WEIRDO, which makes it easier to remember them.

When an adjective clause describes something that is known to exist, we use the indicative. When the adjective clause describes something that is uncertain or unknown, we use the subjuntivo:

Certainty → Indicativo	Uncertainty → Subjuntivo
Necesitas el libro que tiene información sobre Español. He needs the book that has information about Spanish.	Necesitas un libro que tenga información sobre Español. He needs a book that has information about Spanish.
Busco los documentos que describen el contracto. I'm looking for the documents that describe the contract.	Busco documentos que describan el contracto. I'm looking for documents that describe the contract.

When the adjective clause describes a negative expression (nadie, ninguno/-a), the subjuntivo is used:

Certainty → Indicativo	Uncertainty → Subjuntivo
Tengo dos hermanas que viven en Alemania. I have two sisters who live in Germany.	No tengo ninguna hermana que viva en Alemania. I don't have any sisters who live in Germany.

Adverbial clauses are often introduced by conjunctions. Depending on the context in which the conjunctions are used, some may require the subjuntivo while others can be followed by both the subjuntivo or the indicativo.

The following conjunctions require the subjuntivo since they express actions or states that uncertain or have not yet happened:

Cinjunction	Example
a menos que unless	Carlos no va a menos que Juan venga. Carlos is not going unless Juan comes.
sin que without, unless	No puedo hacer nada sin que ellos lo sepan. I can't do anything without them knowing it.
para que so that	Estoy aquí para que me hables. I am here so that you can talk to me.
con tal (de) que provided that	a menos que unless
antes (de) que before	Tengo que comer mi pizza antes que mis hermanas la coman. I have to eat my pizza before my sisters eat it.
en caso (de) que in case	Antonio se prepara en caso de que sus padres necesiten ayuda. Antonio is getting ready in case his parents need help.
hasta que until	Tienes que quedar hasta que lo hayas terminado. You have to stay until you have finished it.