Перед ужином я завернул рыбу в фольгу.

Because the preposition перед normally takes the instrumental case when it means before or in front of.

So:

• ужин = dinner
• перед ужином = before dinner

For a masculine noun like ужин, the instrumental singular ending is typically -ом, which gives ужином.

It can mean both:

• spatially: in front of
• temporally: before

In this sentence, it is clearly temporal:

• Перед ужином = Before dinner

If it were spatial, it would refer to physical position, like перед домом = in front of the house.

Завернул is the past tense, masculine singular, perfective form of завернуть.

Here it means wrapped up / wrapped completely. The perfective aspect shows the action as completed: the fish ended up wrapped in foil.

So the sentence presents the action as a finished event: Before dinner, I wrapped the fish in foil.

The imperfective partner is usually заворачивать.

Compare:

• я завернул рыбу в фольгу = I wrapped the fish in foil
  → completed action
• я заворачивал рыбу в фольгу = I was wrapping / used to wrap / had been wrapping the fish in foil
  → process, repetition, or background action

So in your sentence, завернул is chosen because the speaker is talking about one completed act.

Because рыба is the direct object of the verb завернул.

The speaker did the action to the fish, so Russian uses the accusative:

• nominative: рыба
• accusative: рыбу

This is very common with transitive verbs:

• читать книгу = to read a book
• есть рыбу = to eat fish
• завернуть рыбу = to wrap the fish

Because в + accusative is often used when there is an idea of putting something into a new state or enclosure.

Here, the fish is being wrapped into foil as a result of the action:

• в фольгу = into foil / in foil as the result of wrapping

By contrast, в фольге would describe location or state:

• рыба в фольге = the fish is in foil

So:

• завернул рыбу в фольгу = wrapped the fish in foil
• рыба лежит в фольге = the fish is lying in foil

With this exact verb, в фольгу is the most natural pattern:
завернуть что во что = to wrap something in something

A different verb can use instrumental:

• обернуть рыбу фольгой = wrap/cover the fish with foil

So the choice is partly about verb pattern, not just literal meaning. With завернуть, в фольгу is the expected construction.

Russian often can omit subject pronouns, but here я is helpful.

In the past tense, Russian verbs show gender and number, but not person:

• завернул = masculine singular past
  This could mean I wrapped or he wrapped, depending on context.

So if you remove я, the sentence could be ambiguous unless the context is very clear:

• Перед ужином завернул рыбу в фольгу.
  = Before dinner, I/he wrapped the fish in foil.

That is why я is perfectly natural here.

In the Russian past tense, the verb agrees with the subject in gender and number.

So:

• я завернул = I wrapped (speaker is male)
• я завернула = I wrapped (speaker is female)
• мы завернули = we wrapped

Your sentence uses завернул, so it suggests a male speaker.

Yes. Russian word order is flexible, and changes mostly affect emphasis, not the core meaning.

Your sentence: Перед ужином я завернул рыбу в фольгу.

Other possible versions:

• Я завернул рыбу в фольгу перед ужином.
• Рыбу я завернул в фольгу перед ужином.

The original starts with Перед ужином, which sets the time frame first: As for before dinner...

So the word order is natural, and it gives slight emphasis to when the action happened.

Russian has no articles like a, an, or the.

Whether рыбу means a fish, the fish, or simply fish depends on context. In this sentence, English will usually translate it as the fish, because it sounds like a specific fish the speaker was preparing.

So Russian leaves definiteness unstated unless context makes it clear.