No tengo ni idea de lo que estará pensando ese chico.

why the future “estará” here instead of “esté”?

1 Like

In short, because you’re describing something real (i.e. not knowing something), not an opinion, or an emotion or a hypothetical.

1 Like

makes sense, but why the future tense instead of present?

2 Likes

That’s a way of expressing conjecture or supposition.

3 Likes

By the way, this is an indirect question; compare with “¿Qué estará pensando?”. Using the subjunctive would just not make sense.

1 Like