It is all about the intention.
We know that the Indicative expresses a statement about a fact (affirm or suppose), while subjunctive expresses a virtual idea, something that we can't or don't want to express as a fact.
So, in this realm of the not real, not tangible, not facts, we have the intentions.
An intention is something we want, we aim at, we desire but it is not real, tangible or a fact yet. It lives in our mind or in our heart. For these ideas that live in our mind but we want to materialise into reality, we use subjunctive.
Have a look at this example:
- Juan quiere que vengas. (Juan wants you to come).
You coming is not a reality yet, is something in Juan's heart. It is his desire, therefore we use subjunctive.
We also know that after "querer que" we always use subjunctive, so this is an easy one.
But, what do we do with a construction like "decir que"? Do we use subjunctive by default? Do we use indicative?
Well, it depends. It depends on what we want to say, in our intention.
Look at these examples:
- Mi madre dice que soy buena.
- Mi madre dice que sea buena.
Both are correct but one goes with indicative and the other with subjunctive. And why the hell is that????
In the first one, the mum is stating a fact. She is saying that I am a good girl.
My mum says that I am good.
Whereas in the second one, she is saying that I have to be a good girl.
My mum tells me to be good.
See? The intention is what changes everything. To express the fact (the reality of me being a good girl) we use indicative. To express her desire for me to be good (it's not real yet, it's only a desire in her mind) we use subjunctive.
And, what do we do in questions? We use indicative because we are asking about a fact.
- Declarar (to state): Digo que eres valiente.
- Preguntar (to ask for information): Digo que si eres valiente.
- Pedir (to demand): Digo que seas valiente.
I wrote a bit already about the uses of Indicative and Subjunctive. You can watch the video here.