Use of Cuando with subjunctive and Present/Future Indicative

Question

Both of these sentences are talking about a future event so the present subjunctive is used after cuando. The second clause in one is future indicative and the other is present indicative. What is the difference? Is the difference the close future (go to the store today) vs. the distant future (be a doctor in ten years)? Or the probabilility of occuring more the issue as to which to use present indicative or future indicative.

Cuando sea mayor, seré médico.When I am older, I will be a doctor.Cuando vayas al mercado compra fruta y verdura.When you go to the market, buy fruit and vegetables.

Also, when the subjunctive is used is it always part of the cuando clause?

Accepted Answer

Hola James 
In both sentences it is the same case; the clauses with cuando are using the present subjunctive because that action is still to happen, as opposed to an action that does normally take place:
Cuando sea mayor... (When I'm older [in the future, I'm not older yet] 
Cuando vayas al mercado... (When you go to the market [in the future, maybe tomorrow or next Monday, whenever that may happen...]
It doesn't matter that the action is closer in the future or further away, it's always present subjunctive after cuando if that has not happened yet.
What you have that is different in those two sentences is their main clause. The first one is using the future tense and the other one is using the imperative.

Cuando sea mayor, seré médico. 
When I'm older, I will be a doctor. (future tense)
Cuando vayas al mercado, compra fruta y verduras.
When you go to the market, buy fruit and vegetables.  (imperative, command, order: buy fruit and veg!)

Despite the different tense, they are both referring to something that is still to happen, a future event, i.e. he will be a doctor in the future/ he will buy fruit and veg. The imperative is a command and a command is an action to be done (it hasn't happened yet) 
I hope this clarified your doubt.
Saludos
Inma