Okay, it is very simple and there is an easy way to remember how to do this! The subjunctive is a 'mood', not a tense nor a verb. There are a few subjunctive triggers and there is an acronym that will help you remember! So far, you have been taught the 'indicative' mood.
So, whenever each of these come up in a sentence or if you are thinking of a verb conjugation, consider whether the verb is a form of :
a Wish
an Emotion
an Imperative
a Recommendation
a Doubt
you see Ojalá in the sentence.
Whenever any of these come up, you must use the subjunctive conjugation.. Now, I will teach you how to do that..
Any questions?..