Learn Hardcore Spanish (Spain): Cuando era niña, mi hermana vivía en otra ciudad y solía escribir cartas. - When she was a girl, my sister lived in another city and used to write letters.

Era (imperfect of ser) describes an ongoing background state or life stage in the past: being a child. It’s the normal choice in time-setting clauses like Cuando era niña.
Fui (preterite of ser) would present “being a girl” as a completed, bounded event, which sounds odd here.
Estaba (imperfect of estar) isn’t used for essential identity/life stage. You don’t say estaba niña; you use ser: era niña.
Compare: Cuando era niña… = “When I was a girl…”. For specific age you’d use tener: Cuando tenía ocho años…

Who does era niña refer to: me or my sister?

By default, it refers to the speaker: “When I was a girl…”. The main clause then introduces a different subject (mi hermana). To make this crystal clear, you can say Cuando yo era niña, mi hermana…. If you meant your sister’s childhood, you would say Cuando mi hermana era niña, vivía…

Why is vivía used instead of vivió?

Vivía (imperfect) presents the living situation as ongoing/background during that period.
Vivió (preterite) would present it as a completed, bounded stint, often with a sense of start–end: Mi hermana vivió dos años en otra ciudad (she lived there for two years). Here, the focus is on the background: while I was a girl, my sister was living elsewhere, so the imperfect fits.

Why solía escribir instead of just escribía?

Both can express past habit. Escribía cartas = “she used to/was writing letters (habitually)”.
Soler + infinitive adds an explicit sense of “it was her custom/tendency.” Solía escribir cartas often feels more “habit-leaning” than plain imperfect.
Neither implies a single event; both are habitual in past. Choice is nuance/style.

Is soler common in Spain, and what tenses does it appear in?

Yes, soler is very common in Spain to express habit.
Most frequent forms: present (suelo, sueles, suele…) and imperfect (solía, solías, solían…).
Other tenses like the simple preterite (solió) or future (solerá) are possible but very rare in modern usage.

Who is the subject of solía escribir here?

It’s still mi hermana. In coordinated clauses with y, the subject carries over unless you change it explicitly. If you wanted to switch back to the speaker, you’d say …y yo solía escribir cartas.

Why is there no article in era niña? Could I say era una niña?

With ser + noun indicating category/role (professions, life stages, etc.), Spanish often omits the article: Era niña, Era profesora.
Era una niña is also correct; it can sound a bit more specific or descriptive. If you modify the noun, you normally use the article: Era una niña muy curiosa.

What if the speaker is male (or I want a different wording)?

Male speaker: Cuando era niño…
Very common in Spain regardless of gender: Cuando era pequeño/pequeña… or simply De pequeño/De pequeña…

Can I replace Cuando era niña with De niña or Cuando era pequeña?

Yes. In Spain, De niña and Cuando era pequeña are very natural alternatives, with the same meaning and tone.

Is the comma after the cuando-clause necessary?

When a subordinate clause comes first, Spanish conventionally uses a comma: Cuando era niña, … It’s standard and recommended.

If I move the cuando-clause to the end, does the meaning change?

Often yes.

Cuando era niña, mi hermana vivía… is read as “When I was a girl, my sister lived…”
Mi hermana vivía… cuando era niña will typically be read as “When she (my sister) was a girl, she lived…” To keep “I” as the reference when the clause is at the end, make it explicit: Mi hermana vivía… cuando yo era niña.

Why en otra ciudad and not a otra ciudad?

Vivir en: you live in a place.
Mudarse a: you move to a place. So location uses en: vivía en otra ciudad.

Can I say en una otra ciudad?

No. Otra already carries the “another” meaning, so you don’t use una before it. Say en otra ciudad. If it’s “the other city” (a specific, known one), use en la otra ciudad.

Why do vivía and solía have accent marks?

The acute accent marks the stressed syllable and distinguishes forms:

vivía = vi-VÍ-a (imperfect). Without the accent, the stress would be wrong.
solía = so-LÍ-a (imperfect). Nearly all -er/-ir imperfect forms carry this accent: tenía, comía, escribía.

Why does mi in mi hermana have no accent, while mí sometimes does?

mi (no accent) = the possessive adjective “my”: mi hermana.
mí (with accent) = the prepositional pronoun “me”: para mí, de mí. Here it’s the possessive, so no accent.

Shouldn’t cuando trigger the subjunctive?

Only in certain cases:

Future/uncertain: Cuando sea mayor, estudiaré… (subjunctive).
Past real/habitual background: Cuando era niña… (indicative imperfect). So the indicative is correct here.

Does carta mean “letter” (mail) or “letter” (alphabet)?

carta = a letter you send (mail). Also “menu” in restaurants in Spain.
letra = a letter of the alphabet (and also “lyrics”). So escribir cartas = “write letters (to people).”

Is se solía escribir correct here?

No. Se solía escribir cartas would be odd/misleading (and could read as “letters used to be written” in an impersonal passive). You want a straightforward active subject: (Ella) solía escribir cartas. If you add recipients, use an indirect object: solía escribir cartas a su abuela.

Could I use the progressive: estaba viviendo?

You could, but it changes the nuance. Estaba viviendo highlights an action in progress at a particular moment; vivía is the natural choice for general background/habit. In this sentence, vivía sounds more idiomatic.

How do I say “she used to write me letters”?

Both placements of the clitic are fine:

Solía escribirme cartas.
Me solía escribir cartas. With a specific recipient: Solía escribirle cartas a mi abuela / Le solía escribir cartas a mi abuela.