The Subjunctivisor!

Does cuando require the subjunctive?

It depends. When the main clause is a command or in the (potential) future, cuando requires the subjunctive:

Cuando estés triste, llámame.
When you feel sad, call me.

Cuando estés triste, podrás llamarme.
When you feel sad, you’ll be able to call me.

But cuando takes the indicative when the action in the subordinate clause is either habitual or in the past:

Cuando está triste, llama a su mamá.
When she feels sad, she calls her mom.

Cuando estaba triste, llamó a su mamá.
When she felt sad, she called her mom.

The Subjunctivisor!  |  Subjunctive lesson