"Ven cuando quieras" or "Ven cuando quieres"
On my exam I had this sentence: Ven cuando .... a nuestra casa. Siempre serás bienvenido.
So I chose quieres, but it turned out to be quieras. Why it is in Modo Subjuntivo?
2 Answers
Aileribil's answer is very good and totally correct. I'll just mention another way to think about this.
Ven cuando quieras is a shorthand way of saying, in Spanish, come whenever you want. You don't know when they want to come or even if they will come making this a potentially hypothetical event. Both of these factors contribute to the use of the subjunctive in Spanish. (uncertainty and hypotheticality)
A further example may help you to understand this better. I'll use como (as) instead of cuando (when) because with cuando, Aileribil's answer is the best way to think about it.
Hazlo como quieras.
Do it however you want. Literally, do it as you might want to.
You don't know how they want to do it and you don't care...you leave it up to them.
Hazlo como quieres.
Do it as you wish.
The person has already told you how they want to do something. You know how they want to do it, hence you use the indicative because there is no uncertainty in your mind regarding how they want to do it. You are simply telling them that the way they want to do it is OK with you.
Cuando + subjunctive is used when we want to indicate something that hasn't already happened. "Ven cuando quieras" (the person hasn't been to their house yet).
In this other example:
¿Recuerdas cuando viniste a casa?
Cuando is followed by the indicative mood because that person has already been in their house, it's an action that was completed in the past.
