How do you know when do use the subjunctive?

The subjunctive mood in Spanish is used to talk about desires, wishes, doubts and possibilities. In other words, it's used to talk about things that are not completely certain. It is also used when two clauses are referring to the same subject: for example: 'Espero que yo pueda terminar mi trabajo'.  In addition, the subjunctive is used in sentences where there are two verbs in a sentence; the indicative is used in the main clause while the subjunctive is used in the secondary clause. Lastly, it is important to look out for relative pronouns (e.g. 'que') that link the main clause (where the indicative is used) to the secondary clause (where the subjunctive is used).