Learn Hardcore Russian: Моя сестра почистила лук и морковь, а потом улыбнулась и сказала, что суп будет лучше. - My sister peeled an onion and a carrot, then smiled and said the soup would be better.

Because Russian adjectives/pronouns agree with the noun in gender, number, and case. Сестра is feminine singular nominative, so you use feminine nominative моя.
(Compare: мой брат = “my brother” (masc.), моё окно = “my window” (neut.))

Why does почистила end in -ла?

That’s the past tense feminine singular ending. In the past tense, Russian verbs agree with the subject’s gender/number:

он почистил (masc.)
она почистила (fem.)
они почистили (plural)

What’s the difference between почистила and чистила?

It’s mostly aspect:

почистила (perfective) = she cleaned/peeled (and finished)
чистила (imperfective) = she was cleaning/used to clean (process, no focus on completion)
Here, the story is a sequence of completed actions, so perfective fits.

Why are лук and морковь not in some special form after почистила?

They are in the accusative (direct object), but for inanimate masculine nouns like лук, and many inanimate feminine nouns like морковь, the accusative often looks the same as the nominative.
So почистила лук и морковь = “peeled/cleaned the onion and the carrot(s).”

Does лук mean “onion” or “bow”?

Both are possible in Russian:

лук = onion
лук = bow (weapon)
Context decides. Since it’s followed by морковь and later суп, it’s clearly onion here.

Why use а потом instead of just и потом?

Both can work, but а often signals a shift/contrast or a new step in the narrative (“and then/and after that”), a bit like moving to the next scene.
И потом is more neutral “and then,” while а потом can feel like “and then (after that),” slightly more segmented.

Why is there a comma before а потом?

Because it joins two independent parts:
1) Моя сестра почистила лук и морковь
2) а потом улыбнулась и сказала...
When а connects clauses like this, Russian normally uses a comma.

Why is there a comma before что?

Because что introduces a subordinate clause (reported content):
сказала, что суп будет лучше = “said that the soup would be better.”
In Russian, subordinate clauses are typically separated by commas.

How does reported speech work here—why no quotation marks?

Russian often reports speech in an indirect way with сказать + что (like English “say that…”). Direct quotes are also possible, but then you’d change punctuation and sometimes word order. For example (one possible direct version):
...улыбнулась и сказала: суп будет лучше.

Why is it суп будет лучше and not something like “will be better than …”?

Russian often uses a comparative without explicitly stating “than …” when it’s understood from context. Лучше here means “better” in a general sense: “the soup will turn out better.”
If you do state “than,” you typically use чем: лучше, чем вчера = “better than yesterday.”

Why is лучше used instead of хороший/хорошо?

Лучше is the comparative form (“better”). In суп будет лучше, it functions as a predicate describing the future result: “the soup will be better.”
It’s not modifying a noun directly as an adjective in front of it (like лучший суп = “a better/best soup” depending on context).

Is the word order fixed? Could I rearrange parts of the sentence?

Russian word order is flexible, but changes emphasis. The given order is neutral storytelling. For example, you could move потом earlier for emphasis on sequence:
Моя сестра почистила лук и морковь, потом улыбнулась и сказала...
Still natural, just a slightly different rhythm/focus.

Where is the stress in some tricky words in this sentence?

Common stress patterns here:

сестра́
почисти́ла
морко́вь
улыбну́лась
сказа́ла
бу́дет
лу́чше