Learn Hardcore Ukrainian: Вона майже прийшла, але раптом зупинилася. - She almost arrived, but suddenly stopped.

It’s the reflexive clitic. Зупинитися means “to stop (oneself), to come to a stop,” i.e., an intransitive/middle-voice verb. Without it, зупинити is transitive, “to stop something/someone”:

Вона раптом зупинилася. She suddenly stopped.
Вона раптом зупинила машину. She suddenly stopped the car.

Are both verbs perfective? Why choose perfective here?

Yes. Прийшла (from прийти) and зупинилася (from зупинитися) are perfective. Perfective highlights a bounded, completed event or its sudden onset. With раптом (suddenly), Ukrainian strongly prefers perfective. Imperfective alternatives (приходила, зупинялася) would suggest habitual/ongoing actions and sound off in this context.

Can I say майже приходила instead of майже прийшла?

Generally no. With “almost,” Ukrainian uses perfective to mean “nearly did (but didn’t).” Use:

Вона майже прийшла. She almost arrived. (did not arrive) Compare:
Вона ледь/ледве прийшла. She barely managed to come. (she did come)
Вона ледь не прийшла. / Вона мало не прийшла. She almost came. (didn’t come)
Вона майже не прийшла. She almost didn’t come. (she did come, but nearly didn’t)

Where should I put майже and раптом?

Майже usually goes right before what it modifies: майже прийшла, майже всі, майже готовий. Postverbal прийшла майже is unnatural.
Раптом most naturally comes before the verb: раптом зупинилася. You can also put it at the end: зупинилася раптом, or at the start of the clause: Раптом вона зупинилася—all are grammatical, with slight differences in emphasis.

Do I need a comma before але?

Yes. A comma is required before але (“but”) when it connects two clauses: … прийшла, але …. The adverb раптом is not set off by commas here.

Can I omit the pronoun вона?

Yes. Ukrainian often drops subject pronouns when context is clear because the verb shows person/number (and in the past, gender). Here you can say: Майже прийшла, але раптом зупинилася. Including вона can add clarity or emphasis.

Can I replace але with another word?

та can mean “but” in literary style: … прийшла, та раптом зупинилася.
проте, однак are more formal/bookish: … прийшла, проте/однак раптом зупинилася.
а is weaker (“and/whereas”) and is less suitable for a sharp contrast here.

What’s the difference between прийшла, йшла, підійшла, and прибула?

прийшла: she arrived/“came (to a destination).”
йшла: she was going/walking (process, no implied arrival).
підійшла: she approached/came up to (someone/something).
прибула: she arrived (formal/official, e.g., trains, official visits).

How would I add a destination?

Use typical patterns:

To home: додому — Вона майже прийшла додому, але раптом зупинилася.
To a place: в/у + Accusative or на + Accusative — в офіс, у місто, на станцію
To a person/institution: до + Genitive — до лікаря, до школи All of these can follow прийшла.

How is the sentence stressed and pronounced?

Main stresses:

Вона́ ма́йже прийшла́, але́ ра́птом зупини́лася. Notes:
прийшла́ has stress on the last syllable.
ра́птом stress on the first syllable.
зупини́лася stress on ни́.
The cluster йш in прийшла sounds like y + sh.

Is зупинилась also correct, or must it be зупинилася?

Both occur. -ся is the full form; -сь (→ зупинилась) is a shorter, more colloquial/poetic variant used for euphony, especially after vowels. In careful writing, prefer зупинилася, but you will hear and see зупинилась too.

Could I say але зупинилася раптом instead of але раптом зупинилася?

Yes. Both are grammatical. … раптом зупинилася is the default, but … зупинилася раптом is fine and can slightly emphasize the result (“stopped”) before noting that it was sudden.

Why is there no auxiliary like “was” in the past (e.g., “was stopped”)?

Ukrainian past tense normally uses a single past form (the so-called “-в/-ла/-ло/-ли” forms) without an auxiliary: прийшла, зупинилася. Forms with бути (“to be”) as a past auxiliary exist historically and for special nuances (pluperfect), but in modern standard usage you simply say: Вона … зупинилася, not “вона була зупинилася.”