Learn Hardcore Spanish (Spain): No quiero un anillo caro, solo que la persona que me lo prometa sea sincera. - I don’t want an expensive ring, only that the person who promises it to me is sincere.

No quiero un anillo caro, solo quiero que la persona que me lo prometa sea sincera.
= I don’t want an expensive ring; I just want the person who promises it to me to be sincere.

Spanish often omits a repeated verb when the meaning is clear. So:

No quiero X, solo (quiero) Y.

Both versions are correct:

solo que la persona… (elliptical, more colloquial/natural in speech)
solo quiero que la persona… (explicit, slightly clearer for learners)

Meaning-wise, they are the same here.

Why is prometa used instead of promete? What form is prometa?

Prometa is present subjunctive, 3rd person singular of prometer:

yo prometa
tú prometas
él / ella / la persona prometa
nosotros prometamos, etc.

It’s subjunctive because “la persona que me lo prometa” describes a hypothetical / not-yet-known person, not someone specific that we already know.

Compare:

Conozco a la persona que me lo promete.
I know the person who promises it to me. → a real, known person → indicative (promete)
Quiero a una persona que me lo prometa.
I want a person who would promise it to me. → a desired, not-yet-identified person → subjunctive (prometa)

In the original sentence, la persona que me lo prometa is “whoever that person may be,” so prometa (subjunctive) is required.

Why is it sea sincera and not es sincera?

Sea is present subjunctive of ser.

After verbs of want, desire, preference, fear, etc., Spanish requires the subjunctive in the subordinate clause:

Quiero que sea sincera.
I want her to be sincere.

Using es sincera would sound like a statement of fact about a known person:

La persona que me lo promete es sincera.
The person who promises it to me is sincere. (I know who she is; I’m stating a fact.)

But in the original, the speaker is talking about a wish / requirement:

I don’t want an expensive ring; I just want the person (whoever it is) to be sincere.

Because it’s a wish about something not yet real or not confirmed, Spanish uses sea (subjunctive), not es (indicative).

Why is it sincera (feminine), even though “the person” might be a man?

In Spanish, persona is grammatically feminine, no matter whether we’re talking about a man or a woman. Adjectives must agree with the grammatical gender of the noun:

una persona alta, sincera, amable
esa persona es muy seria

So:

la persona sea sincera
the person is sincere → adjective agrees with persona (feminine), not with the biological gender of the person.

Even if the speaker is thinking of a man, standard Spanish still says:

una persona sincera (not sincero, if the noun is persona)

If you want agreement with the actual gender of the person, you normally change the noun, e.g.:

el hombre que me lo prometa sea sincero
la mujer que me lo prometa sea sincera

What do the pronouns me lo refer to in que me lo prometa? Why do we need both?

Spanish prometer works like “to promise [something] to [someone]”:

prometer algo a alguien

Here:

lo = direct object pronoun = “it” → refers to un anillo (caro)
me = indirect object pronoun = “to me”

So:

que me lo prometa = that (the person) promises it to me

Both are needed if you want to express who receives the promise:

me → to me
lo → the ring

About the order:

With a conjugated verb, indirect comes before direct: me lo prometa
With an infinitive or gerund, they can attach to the end:
prometerme lo / prometiéndomelo (normally written prometérmelo, prometiéndomelo)

Why is it la persona if we aren’t talking about one specific, known person?

La persona here is being used in a generic or unspecified sense:

la persona que me lo prometa
≈ the person who (whoever it may be) promises it to me

Spanish often uses el/la + noun + que… to talk about “the kind of person/thing that…”:

Quiero al profesor que explique bien.
I want the (kind of) teacher who explains well.

Even though the article la is there, context shows we don’t have a concrete, identified individual yet.

You could also express it with alguien:

…solo que alguien que me lo prometa sea sincero/a.

But la persona que… sounds slightly more formal or neutral than alguien que….

Why is it un anillo caro and not un caro anillo?

In Spanish, most descriptive adjectives usually go after the noun:

un anillo caro – an expensive ring
una casa grande – a big house

Putting caro before the noun (un caro anillo) is:

grammatically possible but
sounds literary / poetic / very marked, and often changes the nuance.

With many adjectives, including caro, position can affect meaning or tone:

un anillo caro – simply expensive (high price)
mi caro amigo – “my dear friend” (affectionate, not about money)

So un caro anillo could sound odd, old-fashioned, or be read as “dear ring” in a stylistic sense. The normal, everyday choice for price is anillo caro.

Is solo here an adverb (“only”) or an adjective (“alone”)? How can I tell?

Here solo is an adverb meaning “only / just”.

Clues:

It is modifying the clause that follows, not a noun:
- solo que la persona… → only that the person…
As an adjective, solo/sola would agree with a noun and usually mean “alone / by oneself”:
- una persona sola – a person who is alone
- estoy solo – I’m alone (masc.)
- estoy sola – I’m alone (fem.)

In the sentence:

solo que la persona…
There is no noun after solo for it to modify as an adjective, so it must be the adverb “only/just”.

What about the accent: should it be solo or sólo here?

Modern standard spelling (recommended by the RAE) is:

solo (without an accent) almost always, including this case.

Historically, sólo (with an accent) was used when it meant “only” to distinguish it from solo “alone”. Today, the official guideline is:

Don’t write the accent, except in very rare cases of real ambiguity.

Here there is no ambiguity—it can only mean “only / just”, so:

No quiero un anillo caro, solo que la persona… ✅
…sólo que la persona… is still seen, but it’s not necessary and is considered old-fashioned by many writers.

Could I use the indicative and say la persona que me lo promete es sincera instead? What would be the difference?

Yes, that sentence is grammatically correct, but it means something different.

No quiero un anillo caro, solo que la persona que me lo prometa sea sincera.
→ The speaker is stating a wish / requirement about a not-yet-known or hypothetical person.
→ “I don’t want an expensive ring; I just want the person who would promise it to me to be sincere.”
La persona que me lo promete es sincera.
→ The speaker is talking about a specific person who actually exists and is known, and is simply stating a fact:
→ “The person who promises it to me is sincere.”

So:

Subjunctive (prometa / sea sincera) → desired, hypothetical, not confirmed.
Indicative (promete / es sincera) → real, known, factual.

If prometa and sea are present subjunctive, why do they seem to refer to the future?

Spanish doesn’t really use a separate “future subjunctive” in modern speech; the old forms (e.g., prometiere) are now archaic.

So Spanish normally uses the present subjunctive for:

actions or states that are uncertain / hypothetical / desired and
are usually in the future relative to the main verb.

Here:

No quiero… solo que… me lo prometa / sea sincera.

Those things (promising the ring / being sincere) will happen later, if they happen at all, so English naturally uses future-like structures:

…who will promise it to me…
…who is (will be) sincere…

But in Spanish, this is simply done with the present subjunctive:

prometa, sea sincera → grammatically “present subjunctive,” but often future in meaning.