Learn Spanish (Latin America): ¿Dónde estará la poción? Ojalá yo fuera invisible. - Where could the potion be? I wish I were invisible.

Fuera is the imperfect (past) subjunctive of ser (to be). We use the subjunctive here because ojalá (I wish / if only) expresses a strong desire. To form these irregular past subjunctives, you take the ustedes/ellos form of the preterite past tense (fueron), drop the -on, and add the imperfect subjunctive endings. So, fuer- + -a gives us fuera.

Why does the sentence use the future tense estará if the speaker is looking for the potion right now?

In Spanish, the future tense is frequently used to express a guess, wonder, or probability about the present. So while ¿Dónde estará la poción? translates literally to 'Where will the potion be?', it actually means 'Where could the potion be?' or 'I wonder where the potion is right now.'

Why use the past subjunctive (fuera) when they are wishing to be invisible right now?

When you use ojalá with the imperfect (past) subjunctive, it expresses a wish about the present that is unlikely, contrary to reality, or impossible. Saying Ojalá yo fuera invisible means 'I wish I were invisible (right now, but I am not).' Notice that English does the exact same thing: we say 'I wish I were' (a past tense form) to talk about a present dream.

Since being invisible is a temporary state caused by a potion, why use ser (fuera) instead of estar ()?