Hello there.

You’re right.
Relative clauses in English always take the indicative mood; well, unless they take a modal, for example, but then there is no mood at all. If we’re talking about mood, only the indicative is possible. This is indeed problematic when you have to translate into Spanish, because in Spanish relative clauses can take either mood, with a difference in meaning. This difference, in English, is usually inferred from context, and sometimes it’s just a mater of guessing, unfortunately, or of figuring out what makes more sense.

I’ll give you an example:
"She wants to buy a house that has a swimming-pool."

This sentence (with the verb in the clause in the indicative mood) has two possible translations in Spanish, with different meanings:

1. Ella desea comprar una casa que tiene una piscina. (tiene = indicative)
2. Ella desea comprar una casa que tenga una piscina. (tenga = subjunctive)

In the first sentence, she has already seen the house she wants to buy, and it happens to have a swimming-pool.
In the second, she hasn’t chosen a specific house yet, but she knows she wants one with a swimming-pool. She will buy a house if she finds one that has a swimming pool.

Unfortunately, there is no way, using a relative clause, to show this difference in English. If you have to translate a similar sentence from English into Spanish, you will have to examine the context of the sentence for hints about its meaning.

Good luck!

EDITED to clarify:
Let me explain what happened here and see if I can help.
Melinda fell prey of the "that trap", as I call what I'm going to explain now. She confused relative clauses with something called "that-clauses", perhaps because of the introductory word in both types. But that’s the only thing they have in common. This is a mistake many students of English make; it doesn’t seem easy for them, even at the upper-intermediate and advanced levels, to see the difference between both types of clause. The ones called "that-clauses" are nominal clauses.

The clauses in my examples are relative clauses, or adjectival clauses. They function in the way a predicative adjective would, postmodifying a noun. That’s their role. The introductory “that” is a relative pronoun and it ALWAYS has a syntactic function in the clause.

The clauses Melinda posted, and which she incorrectly calls “relative”, are in reality nominal clauses (or noun clauses). Nouns and adjectives don’t function in the same way; neither do nominal and relative clauses. In Melina's sentences, the clauses are DIRECT OBJECTS, so they can't possibly be relative clauses, since an adjective or a structure with adjectival force can never be a direct object; it’s impossible. Verbs such as insist and demand, which she used, and several others such as suggest, recommend, order, propose and urge, require the use of the marked subjunctive (or some more informal constructions, such as should + infinitive) in that clauses, which are, I insist, nominal clauses. The subjunctive required by the “that” type of nominal clause is also used after expressions such as “It’s necessary”, “it’s commendable”, “it’s imperative”, etc.
One more difference between these and relative clauses is that the word “that”, in nominal clauses, is NOT a relative pronoun but a subordinating conjunction and, also unlike what happens with relative clauses, it’s NEVER part of either the subject of the predicate of the clause.

As you can see, the differences between both clauses are many and clear. Yet, students often fail to distinguish one from the other. I wish I could explain why!

I hope this will help you to avoid confusing both types of clauses.

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