Y tú by Airbag, the Argentine rock band, is a simple, warm promise of loyalty. Almost every line starts with cuando (when), and each one names a bad moment: "Cuando no pueda continuar" (when I can't go on), "Cuando no quiera hablar de amor" (when I don't want to talk about love). The answer is always the same: "yo sé que estarás" (I know you'll be there). Notice the subjunctive after cuando: pueda, quiera, falte. That's the grammar Spanish uses for future moments that haven't happened yet, and this song is a great way to hear it on repeat.
The chorus is the heart of it: "Cuando ya no habrá más nadie, tú estarás" (when there's no one else left, you'll be there). Más nadie is a very Latin American way of saying nadie más (no one else). Then the singer flips it around and promises the same back: "Sabes que estaré contigo" (you know I'll be with you), "Con vos" (with you). Vos is the Argentine word for "you", used instead of tú, so you get both forms in one song. It's not a complicated song, and that's the point: it's the kind of chorus you end up shouting with a crowd.