When should I use the present subjunctive in Spanish?

Whether you use a verb in the subjunctive depends on what the previous verb does to the meaning of the sentence.Verbs in the indicative are used if the sentence describes reality. Verbs in the subjunctive are used 1. if the sentence describes things that are uncertain, and 2. to describe an emotional reaction. WEIRDO is a mnemonic which helps you remember ways in which uncertainty and emotion could come up in a sentence.
W – Wishes e.g. ‘Quiero que compremos un coche nuevo.’ (I want us to buy a new car.) It’s uncertain - we haven’t decided whether or not to buy one yet. I - Impersonal expressions (of the form es _______ que…) ‘Es importante que termines la tarea hoy.’ (It’s important that you finish the task today.) They’re unsure that the task will get done (uncertainty), and worried because it’s very important (emotion). E – Emotions e.g. ‘No me gusta que todos estos coches sean tan caros.’ (I don’t like it that all these cars are so expensive.) They’re annoyed at the price – an emotional reaction.R – Recommendations e.g. Me permite que compre el coche. (She’s letting me buy the car.) She accepts it - an emotional reaction.D – Doubts e.g. No pienso que sea posible. (I don't think it's possible.) He’s uncertain.O – Ojala e.g. Ojalá que venga pronto. (I hope he comes soon.) She’s uncertain as to whether he’ll come – and is emotional about it.