В чае плавает кусочек лимона.

Because в here means in, showing location, and after в with location Russian uses the prepositional case.

• dictionary form: чай = tea
• prepositional singular: чае
• so в чае = in the tea

This is a very common pattern:

• в доме = in the house
• в книге = in the book
• в чае = in the tea

Russian word order is flexible, so starting with В чае sets the scene first: as for the tea / in the tea...

So the sentence structure is roughly:

• В чае = in the tea
• плавает = is floating
• кусочек лимона = a small piece of lemon

This word order sounds natural when you first mention the location and then say what is there. English often does the same in a more limited way: In the tea, there is a piece of lemon floating.

Literally, yes, плавать is the verb used for to swim / to float. But with an object like a lemon piece in tea, the natural meaning is is floating.

So in this sentence:

• плавает does not mean the lemon is actively swimming
• it means it is floating around / bobbing in the tea

Russian commonly uses плавать this way for things in liquid.

Russian present tense often covers both:

• it floats
• it is floating

Russian does not have a separate tense form like the English present continuous. So плавает can mean either, depending on context.

In this sentence, English usually prefers is floating because it describes what is happening in the tea right now.

Кусочек is a diminutive form of кусок.

• кусок = a piece, chunk
• кусочек = a little piece, small piece

Here it sounds natural because a lemon in tea is usually a small piece or slice, not a big chunk. Diminutives in Russian often express:

• small size
• natural everyday tone
• sometimes a slightly softer or more pleasant feel

So кусочек лимона sounds more natural here than кусок лимона.

Because after a noun like кусочек meaning a piece of, Russian usually puts the next noun in the genitive case.

So:

• лимон = lemon
• лимона = of lemon

That is why:

• кусочек лимона = a little piece of lemon

This is the same pattern as:

• чашка чая = a cup of tea
• кусок хлеба = a piece of bread
• бутылка воды = a bottle of water

Russian has no articles. It does not use separate words for a/an or the.

So кусочек лимона can mean:

• a piece of lemon
• the piece of lemon

The exact meaning depends on context. In this sentence, English would normally say a piece of lemon unless both speakers already know which piece is meant.

Because the grammatical subject is кусочек.

Even though лимона follows it, the main noun is still кусочек:

• кусочек = singular
• therefore плавает = singular

So the sentence is literally structured like:

• A small piece of lemon is floating

The verb agrees with кусочек, not with лимона.

Yes. Russian word order is flexible, although different orders sound slightly different in emphasis.

For example:

• В чае плавает кусочек лимона. = neutral, scene first
• Кусочек лимона плавает в чае. = focuses more on the lemon piece
• Плавает в чае кусочек лимона. = more literary or expressive

All are understandable, but В чае плавает кусочек лимона is very natural if you want to say what is in the tea.

Because the lemon piece is inside the tea, not on it.

• в = in, inside
• на = on, on the surface, at, or in some other contexts

Since tea is a liquid and the lemon is in it, Russian uses в чае.

Чае is pronounced in two syllables, roughly CHA-ye.

A few useful stress notes for the whole sentence:

• в ча́е
• пла́вает
• кусо́чек
• лимо́на

So the full sentence is pronounced approximately:

v CHA-ye PLA-va-yet ku-SO-chek li-MO-na

The е in чае does not disappear; it is not one syllable like English chai.

Not necessarily. Кусочек means a small piece. In context, it could be:

• a small chunk
• a small wedge
• a little bit of lemon
• sometimes effectively what English would call a slice, depending on the situation

If you wanted to be more specific, Russian could use other words, but кусочек лимона is a very natural general phrase.

It is the genitive singular.

The reason is that Russian often uses the genitive after nouns that express:

• quantity
• a part of something
• a container of something

Here кусочек is a part noun, so the thing it is a part of goes into the genitive:

• кусочек чего? = a little piece of what?
• лимона = of lemon

So this is a standard Russian grammar pattern, not something special only for this sentence.