The subjunctive is most often used when expressing volition. Using English and Spanish as examples:
The doctor recommends that you eat vegetables and fruits.
El médico recomienda que comas verduras y frutas.
Sometimes it is used in Spanish, but the indicative is used in English (generally when expressing doubt or emotion):
I doubt that it is so easy.
Dudo que sea tan fácil.
And sometimes vice versa (generally for a hypothetical in present tense using "if"):
If he be alive (archaic)
Si está vivo"
There are other examples, like:
I want him to tell me the truth.
*I want that he tell me the truth. (not that tells would work here either)
Quiero que me diga la verdad.
Here, the English usage of "want" has been completely rewritten to use a different grammatical structure which uses neither the indicative nor subjunctive (but rather the infinitive), and I'm not interested in that.
My question is: How do various languages choose whether to use the indicative or subjunctive under what is otherwise the same grammatical framework? Are there any differences at all among Romance languages? Feel free to bring up anything that suits the topic, be it Latin, non-Indo-European languages, etc. (yes, I just used the subjunctive there).