En cuanto

The Subjunctivisor!

Does en cuanto require the subjunctive?

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

En cuanto hierva el agua, añade sal.
As soon as the water boils, add salt.

En cuanto hierva el agua, podrás añadir sal.
As soon as the water boils, you will be able to add salt.

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

En cuanto hierve el agua, siempre añada sal.
As soon as the water boils, she always adds salt.

En cuanto hirvió el agua, añadó sal.
As soon as the water boiled, she added salt.

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