Why does only Spanish have the subjunctive form?

Question

Actually, English has it as well but it is more noticeable in the formal British English. The subjunctive is used after certain expressions that contain an order or a request, a hypothetical, or a wish.

I suggest that you accept my proposal.

She demanded that he leave immediately.

We ask that you be careful.

Answer 1

The subjunctive is one of three moods in Spanish: indicative, subjunctive and imperative. The indicative is used for stating facts, the imperative is used for orders and the subjunctive mood is used to express emotions, uncertainty or what I would call "general" statements.

There is a mnemonic device I use to help students remember when to use the subjunctive called WEIRDO.

W - Wishes (Deseo que me gradua. I wish to graduate.)

I - Impersonal (Es importante que la gente recicle. It is important that people recycle.

E - Emotions (Ella se siente triste que su equipo favorito haya perdido. She feels bad that her favorite team has lost.)

R - Recommendations (Recomendamos que tú vayas a Madrid. We recommend that you go to Madrid.)

D - Doubts (Dudo que digan la verdad. I doubt they´re telling the truth.)

O - Orders (Mi madre exige que yo limpie el baño. My mom demands me to clean the bathroom.)

There is a few important things to point out about these sentences. In most, the subject between the two clauses (the parts of the sentence that "que" separates) changes. A general "it" (it is important)/la gente, ella/su equipo, nosotros (recomendamos)/tú, yo (dudo)/ellos (digan) and mi madre/yo. The first sentence has yo as the subject before and after "que," the key here is that while I hope or wish that I graduate, whether or not I will is uncertain.

These are all different from the indicative, an example being "Quiero ir al centro comercial." I want to go to the mall. That's a certain, undeniable fact. However, if my mother wants me to go to the mall, the fact that she wants something is indicative because it's a fact, but whether or not I go to the mall is uncertain. I may or may not go. As a result, we have "Mi madre quiere que yo vaya al centro comercial."

As for why Spanish and the other romance languages have this, I'm not sure that that's certain. It's probably a question for a linguist and it's not something I've heard or read anything definitive on. Perhaps at some point, either while Proto-Indo-European existed or after, speakers of what eventually lead to Latin made a distinction between stating facts and expressing emotions or uncertainty. After all, as you pointed out, in English I can say something like "I'm asking you to be careful." The fact that I do or don't know whether or not you will be careful doesn't seem to be something that the language makes any sort of adjustment for.

I hope this helps.