Learn Spanish (Latin America): Cuando mi abuelo se jubile, va a viajar a México. - When my grandfather retires, he is going to travel to Mexico.

Because the retirement hasn't happened yet! When we use cuando (when) to talk about a future event, Spanish requires the subjunctive mood. If we used the regular present tense (jubila), it would mean he retires as a regular, repeating habit, which doesn't make sense here.

Why is there a se before jubile?

The verb for to retire is jubilarse, which is reflexive. Without the se, jubilar means to force someone else into retirement (like a company retiring an employee). With se, it means to retire oneself.

Since jubilarse is an -ar verb, why does it end in an -e here?

That is how the present subjunctive is formed. You take the regular verb stem and swap the vowel: -ar verbs take -e endings, while -er and -ir verbs take -a endings. Since mi abuelo is third person singular (he), the ending is -e.

Can I use the regular future tense instead of va a viajar?

Yes! You can say viajará (he will travel) instead of va a viajar (he is going to travel). However, using ir a + infinitive is much more common in everyday Latin American spoken Spanish. Both work perfectly with a cuando + subjunctive phrase.