Learn Hardcore Spanish (Latin America): Cuando comparto mi tristeza con mi hermana, siento más confianza en ella. - When I share my sadness with my sister, I feel more trust in her.

The subject yo is not written, but it’s understood from the form comparto. So comparto is present indicative, 1st person singular, which is required here.

Why is it cuando comparto and not cuando comparta with the subjunctive?

In Spanish, cuando + present indicative is used for:

habitual actions or general truths in the present.

Here, the idea is something like:

Cuando comparto mi tristeza con mi hermana
= Whenever I share my sadness with my sister / When I (usually) share my sadness with my sister…

This is a general, habitual situation, so you use present indicative: comparto.

You would use cuando + subjunctive (comparta) when talking about a future or hypothetical action:

Cuando comparta mi tristeza con mi hermana, me sentiré mejor.
= When I share my sadness with my sister (in the future), I will feel better.

So:

Habitual / general: cuando comparto
Future / uncertain: cuando comparta

Why is it mi tristeza instead of something with triste?

Triste is an adjective meaning sad.
Tristeza is a noun meaning sadness.

Estoy triste = I am sad. (adjective)
Comparto mi tristeza = I share my sadness. (noun)

In the sentence, sadness is an object that you share, so Spanish prefers the noun tristeza, not the adjective triste. You can’t say comparto mi triste; that’s ungrammatical.

Is mi tristeza (my sadness) necessary? Could it just be comparto tristeza?

You can say comparto tristeza con mi hermana, and it’s grammatically correct, but:

mi tristeza is more personal and specific: my own sadness.
tristeza alone is a bit more general: sadness (as a feeling or mood).

So:

Comparto mi tristeza con mi hermana
Emphasizes that you are opening up about your own pain.
Comparto tristeza con mi hermana
Sounds more like we share sadness (we are both sad, or our sadness is shared) and is less common in this emotional, confessional sense.

The original sentence with mi tristeza feels more natural for talking about opening up emotionally.

Why is it con mi hermana and not a mi hermana?

The verb compartir usually takes the preposition con when you mean share something with someone:

Compartir algo con alguien = to share something with someone

So:

Comparto mi tristeza con mi hermana
= I share my sadness with my sister.

Using a mi hermana would change the structure and sound unnatural here. You’d use a with verbs like:

Contarle algo a alguien = to tell something to someone
e.g. Le cuento mis problemas a mi hermana.

What exactly does confianza mean here? Is it the same as English confidence?

Confianza is a false friend. It usually means:

trust, feeling of safety/comfort with someone, or
confidence in the sense of self-assurance (depending on context).

In siento más confianza en ella, the meaning is:

I feel more trust in her
I feel more comfortable with her
I feel I can trust / open up to her more

It’s not about self-confidence here; it’s about trust in another person.

Why is it siento más confianza and not tengo más confianza?

Both are possible and common, but there’s a nuance:

Tengo más confianza en ella.
= I have more trust in her. (More factual, like a state or level of trust.)
Siento más confianza en ella.
= I feel more trust in her. (Emphasizes the subjective, emotional experience.)

In a sentence about emotions and opening up, siento más confianza sounds very natural because it highlights how you feel.

Why is the preposition en used in confianza en ella? Could it be confianza con ella or confianza hacia ella?

With confianza meaning trust in someone, the most standard preposition is en:

Tener / sentir confianza en alguien = to have / feel trust in someone.

So:

Siento más confianza en ella.
= I feel more trust in her.

Other prepositions are possible but change the nuance:

Confianza con alguien
Often suggests familiarity / informality or a relaxed relationship:
- Tengo mucha confianza con mi hermana.
  = We’re very close / we’re very comfortable with each other.
Confianza hacia alguien
Can sound more formal or abstract, like an attitude directed toward someone.

In this sentence, en ella is the natural choice for trust in her.

Can I change the word order and say: Siento más confianza en ella cuando comparto mi tristeza con mi hermana?

Yes, that’s completely correct and very natural.

Spanish allows the cuando clause to come before or after the main clause:

Cuando comparto mi tristeza con mi hermana, siento más confianza en ella.
Siento más confianza en ella cuando comparto mi tristeza con mi hermana.

The meaning doesn’t change. Sometimes speakers choose one order for emphasis or rhythm, but both are fine.

Why is there no yo in the sentence? Could I say Cuando yo comparto mi tristeza…?

Spanish often omits subject pronouns (yo, tú, él, etc.) because the verb ending already shows who the subject is:

comparto → clearly means yo.

You can say:

Cuando yo comparto mi tristeza con mi hermana, siento más confianza en ella.

But:

It adds a small emphasis on yo (I), as in when *I share…*.
In neutral, everyday speech, most speakers would just say cuando comparto… without yo, because it’s understood.