В супе плавала одна фрикаделька, а в тарелке брата их было целых пять.

Because в means in, and here it expresses location, not movement.

When в means in/inside and answers where?, Russian normally uses the prepositional case:

• суп → в супе
• тарелка → в тарелке

If it were movement into something, Russian would usually use the accusative instead:

• в суп = into the soup
• в тарелку = into the plate/bowl

So here:

• В супе = In the soup
• в тарелке брата = in the brother’s plate/bowl

Because the verb agrees with фрикаделька, which is a feminine singular noun.

• фрикаделька is feminine
• past tense verbs in Russian agree in gender and number

So:

• плавал = masculine singular
• плавала = feminine singular
• плавало = neuter singular
• плавали = plural

Since the subject is одна фрикаделька, the correct form is плавала.

Because один changes for gender, just like an adjective.

The noun фрикаделька is feminine, so one must also be feminine:

• один = masculine
• одна = feminine
• одно = neuter

So:

• одна фрикаделька = one meatball

This also slightly emphasizes the idea of just one / a single meatball.

Both are possible in different contexts, but they do not feel exactly the same.

• была would just state existence/location: there was one meatball in the soup
• плавала adds imagery: one meatball was floating in the soup

So плавала is more vivid and descriptive. It makes you picture the meatball actually floating around.

Because а often marks a contrast or comparison, not just simple addition.

Here the sentence is comparing two situations:

• in one soup/plate there was only one
• in the brother’s plate there were five

So а works like:

• whereas
• while
• but

It sets up the contrast very naturally.

Because брата is the genitive singular of брат, and it shows possession.

So тарелка брата literally means:

• the brother’s plate
• or more literally, the plate of the brother

This is a very common way to express possession in Russian.

Compare:

• книга брата = the brother’s book
• машина сестры = the sister’s car
• тарелка брата = the brother’s plate

In the sentence, в тарелке брата means in the brother’s plate/bowl.

Их means them, referring back to фрикадельки.

The full idea is:

• в тарелке брата их было целых пять
• in my brother’s plate, there were five of them

Russian often avoids repeating the same noun if it is already clear from context. Instead of repeating фрикаделек, the sentence uses их.

So this is similar to English:

• There was one meatball in the soup, and in my brother’s bowl there were five of them.

Because Russian uses a special kind of existential/quantity construction here.

Их было пять literally works more like:

• of them, there were five

This is natural Russian when talking about the number of something already understood from context.

By contrast, они были пять would sound wrong here, because пять is not being used as a normal predicate adjective. The sentence is not saying they were five in the English sense; it is saying there were five of them.

So:

• их было пять = natural
• они были пять = not natural here

Because with quantity expressions like пять, Russian very often uses neuter singular было in existential sentences.

This is especially common when the sentence means something like:

• there were five
• there were five of them

So:

• их было пять = there were five of them

This is a very common pattern in Russian. The verb does not always agree with the logical plural in the way an English speaker might expect.

A useful model is:

• На столе было три книги.
• В комнате было пять человек.

Even though the things counted are plural, было is normal in this kind of sentence.

Целых adds emphasis. It means something like:

• a whole
• as many as
• no fewer than

So:

• целых пять = a whole five / as many as five

It makes the contrast stronger: one meatball in one place, but in the brother’s plate there were a whole five.

Because with numerals like пять, words connected to the counted noun often appear in forms associated with the noun that would be in the genitive plural.

If the noun were stated, you would have:

• пять фрикаделек

And an adjective describing those meatballs would also typically appear in the corresponding plural form:

• целых пять фрикаделек

In your sentence, the noun фрикаделек is omitted because it is already understood, but целых stays.

So:

• целых пять = shortened from целых пять фрикаделек

It would be фрикаделек, the genitive plural.

So the full version would be:

• В тарелке брата было целых пять фрикаделек.

This is because after пять and higher numerals, the counted noun normally goes into the genitive plural:

• пять книг
• пять домов
• пять фрикаделек

That is different from one, where the noun stays singular:

• одна фрикаделька

Because Russian word order is flexible, and the speaker often arranges the sentence to highlight what is already known and what is new or important.

Here:

• В супе sets the scene first: in the soup...
• then comes плавала одна фрикаделька
• in the second clause, в тарелке брата also sets the location first
• the really important new information comes at the end: целых пять

This makes the contrast feel natural and dramatic.

So:

• Одна фрикаделька плавала в супе is grammatically possible
• but В супе плавала одна фрикаделька... sounds more like storytelling and comparison

Yes. Depending on context, тарелка can refer to a plate or a bowl-like dish, especially in food contexts.

Since the sentence talks about soup and meatballs, English might naturally translate it as:

• plate
• bowl

The Russian grammar does not change; this is just a vocabulary/context issue.