For any native English speaker, the Subjunctive is very hard to master, given that it is no longer used in the spoken English language! It is very common to make mistakes, but to remember certain triggers, the words and phrases that indicate the subjunctive is needed, there is an anacronym of WEIRDO. W - stands for Wish/Desire - e.g. querer/esperar/desear que. E - stands for Emotion - e.g. alegrar/sorprender que. I - stands for Impersonal Expressions - e.g. es importante/necesario que. R - stands for Reccomendation - e.g. aconsejar/recomendar que. D - stands for Doubt - e.g. dudar que. O - stands for Ojalá - which always takes the subjunctive after it! This is a helpful, but not extensive, guideline on when to use the subjunctive mood, but don't be surprised or dismayed that this doesn't come naturally when it is one of the most difficult things for native English speakers to grasp about Spanish.