Many students can't get their heads around the subjunctive but it's much easier than it seems! A really handy way to remember when to use the subjunctive is by using the accronym 'WEIRDO':
Wishes: When someone expresses what they want someone or something to do something. E.g. Quiero que hagas tus deberes. Other wishing/wanting phrases that are followed by subjunctive are phrases like 'Espero que..' (I hope that) and 'Mandar que...' (to order that sb does something). All of these express a desire for somebody to do something and therefore, use the subjunctive.
Emotion: Whenever an emotion is expressed it is followed by the subjunctive. For example, 'me gusta que haya muchas tiendas en esta ciudad' or 'me enoja que ella nunca pueda ir de vacaciones conmigo'.
Impersonal expressions: Impersonal expressions are usually used to express and opinion or judgement. For example, 'Es extraño que no puedas comer esto.' Some other impersonal expressions that are followed by the subjunctive are 'Es necesario que...', 'Es bueno que...', 'Es malo que...'. However, remember that if the impersonal expression expresses certainty e.g. 'es cierto que' or 'es obvio que', then it is NOT followed by the subjunctive (More on this later).
Recommendations: When you recommend something to someone e.g. 'Recomiendo que no hagas mucho ejercicio durante las vacaciones'.
Doubt/Denial: When you express denial or you express negative thinking e.g. 'Niego que yo tenga que hacer esto.' Or 'No creo que haya un restaurante aquí'. Expressions that express certainty (as mentioned above) e.g. 'es claro que...' are NOT followed by the subjunctive.
Ojalá: This is just one phrase that is relatively common in Spanish. 'Ojalá que' means 'I really hope that...' or 'I hope to God that...' and it is followed by the subjunctive.
There are also a few other phrases that need the subjunctive e.g. 'para que' (so that) e.g. 'Para que tenga mucho dinero, necesito trabajar muy duro' and 'Cuando' (only when referring to the future e.g. 'Compraré un coche cuando sea mayor'.
Also notice that most verbs conjugated in the subjunctive are followed by 'que' as seen in most of the above examples. The subjunctive is not always followed by 'que' but it is another indication that you should be using the subjunctive.
Hope that helps and if any further explanation is needed, feel free to send me a message or book a meet the tutor session!