Для супа я сначала сварила куриный бульон.

Для супа literally means for soup or for the soup.

In this sentence, it means something like for making the soup or as part of preparing the soup. The speaker is explaining the purpose of the action: the chicken broth was made in order to make the soup.

So для here expresses purpose.

Because для always requires the genitive case.

• Nominative: суп
• Genitive singular: супа

So:

• суп = soup
• для супа = for soup / for the soup

This is a very common pattern in Russian:

• для мамы = for Mom
• для работы = for work
• для супа = for soup

This is an aspect question.

• варить = imperfective
• сварить = perfective

Here, сварила is perfective, which means the speaker is talking about a completed result: she finished making the chicken broth.

That fits well with сначала (first), because the sentence describes one completed step in a sequence:

1. first, she made the broth
2. then presumably she did something else to continue making the soup

If you said варила, it would sound more like:

• she was boiling/cooking it
• she used to cook it
• the process itself is being emphasized rather than the finished result

Not always in the narrow English sense.

In cooking, сварить often means to cook by boiling or to prepare by simmering/boiling, and in English it is often translated more naturally as:

• made
• cooked
• prepared

So сварила куриный бульон is most naturally understood as made chicken broth, not just boiled chicken broth.

Russian cooking verbs often sound more specific than the best English translation.

In the past tense, Russian verbs show gender in the singular.

• сварил = a male speaker / he cooked
• сварила = a female speaker / she cooked
• сварило = it cooked
• сварили = they cooked

So я сварила tells you that the speaker is female.

A male speaker would say:

Для супа я сначала сварил куриный бульон.

Yes, it often could be left out.

Russian frequently drops subject pronouns when the meaning is clear from the verb form or the context. So you could also say:

Для супа сначала сварила куриный бульон.

However, я is still perfectly natural here. It may be included:

• for clarity
• because the sentence stands alone
• for slight emphasis on I

So я is not strange here, but it is not always strictly necessary.

Because куриный бульон is the direct object of сварила.

The verb сварить takes a direct object in the accusative case. But here there is an important detail:

• бульон is masculine singular
• it is inanimate

For inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: куриный бульон
• accusative: куриный бульон

That is why the form does not visibly change.

If it were an animate masculine noun, you would see a different form.

Бульон is broth, while суп is soup.

So the sentence describes an earlier step in cooking:

• first she made chicken broth
• then that broth would be used to make the soup

This is why the sentence says Для супа... куриный бульон. The broth is the base for the soup.

Сначала means first or at first.

It shows that this action was the first step in a sequence. The sentence implies that more steps came after that.

So the structure is basically:

• For the soup
• I first
• made chicken broth

It helps organize the events in time.

Yes, Russian word order is flexible.

This sentence begins with Для супа to set the context or purpose right away: as for the soup / for making the soup.

Other word orders are also possible, for example:

• Я сначала сварила куриный бульон для супа.
• Сначала я сварила куриный бульон для супа.

These all express roughly the same basic meaning, but the emphasis shifts a little.

The given version puts the spotlight first on what this was for: the soup.

Yes.

Russian has no articles, so супа by itself does not tell you whether English should use:

• for soup
• for the soup
• sometimes even to make soup

The correct English version depends on context.

That is very normal when translating from Russian to English: Russian often leaves article-type distinctions unstated, and English has to choose based on the situation.