W tym zdaniu rzeczownik jest w liczbie pojedynczej.

Because the preposition w usually requires the locative case when it means in in the sense of location or context.

So:

• to zdanie = nominative, this sentence
• w tym zdaniu = locative, in this sentence

Both words change because tym must agree with zdaniu in case, number, and gender.

Zdaniu is the locative singular form of zdanie.

The noun zdanie is:

• neuter
• usually ends in -e in the dictionary form
• nominative singular: zdanie
• locative singular: zdaniu

That is why after w you get w zdaniu, and with ten you get w tym zdaniu.

Tym is a declined form of ten (this).

It matches zdaniu because:

• zdanie is neuter
• it is singular
• it is in the locative

So:

• nominative: to zdanie = this sentence
• locative: w tym zdaniu = in this sentence

This kind of agreement is very important in Polish: demonstratives, adjectives, and many pronouns have to match the noun they describe.

Because rzeczownik is the subject of the sentence, so it is in the nominative case.

Here, rzeczownik means noun, and it is the thing being described:

• rzeczownik jest... = the noun is...

So there is no need to change it to another case.

Jest is the 3rd person singular form of być (to be) in the present tense.

It is singular because the subject is singular:

• rzeczownik = one noun
• therefore: rzeczownik jest

Compare:

• rzeczownik jest = the noun is
• rzeczowniki są = the nouns are

It is the standard Polish expression for in the singular.

Literally:

• liczba = number
• pojedyncza = single / singular

So liczba pojedyncza means singular number, and:

• w liczbie pojedynczej = in the singular

The matching expression for in the plural is:

• w liczbie mnogiej

Because after w in this expression, Polish uses the locative case.

So:

• dictionary form: liczba pojedyncza
• after w: w liczbie pojedynczej

Both words change:

• liczba → liczbie
• pojedyncza → pojedynczej

The adjective pojedynczej has to agree with liczbie.

It is locative singular.

More specifically:

• liczbie = locative singular of liczba
• pojedynczej = locative singular feminine form of pojedynczy

This happens because the whole phrase follows w:

• w liczbie pojedynczej

Not always.

With w, there are two very common possibilities:

• locative when it means in and describes location or context

  • w domu = in the house
  • w tym zdaniu = in this sentence

• accusative when it expresses motion into

  • wchodzę w wodę = I am going into the water

In your sentence, there is no motion, so Polish uses the locative.

Yes. Polish word order is more flexible than English word order.

For example, these are all possible:

• W tym zdaniu rzeczownik jest w liczbie pojedynczej.
• Rzeczownik w tym zdaniu jest w liczbie pojedynczej.
• Rzeczownik jest w tym zdaniu w liczbie pojedynczej.

They all mean roughly the same thing, but the emphasis changes slightly.

The original version sounds natural if you want to start with the context: in this sentence.

Because Polish does not have articles like English a/an/the.

So:

• rzeczownik can mean a noun or the noun
• zdanie can mean a sentence or the sentence

The exact meaning comes from context. In this sentence, English usually uses the noun and this sentence, but Polish does not need articles to express that.

Yes, indirectly.

Pojedynczy means single, individual, or singular, and it is connected to the idea of one.

So liczba pojedyncza is the grammatical category referring to one item. Its opposite is:

• liczba mnoga = plural

This is why the phrase works well to describe the form of a noun.

Yes, but it means something slightly different in focus.

• W tym zdaniu rzeczownik jest w liczbie pojedynczej.
  Focus: in this sentence

• Ten rzeczownik jest w liczbie pojedynczej.
  Focus: this noun

So the original sentence is about the noun’s form within a particular sentence, while your version points more directly to a specific noun.

Yes, it is very standard.

Polish grammar explanations often use phrases like:

• w liczbie pojedynczej = in the singular
• w liczbie mnogiej = in the plural
• w rodzaju męskim = in the masculine gender
• w czasie przeszłym = in the past tense

So this sentence sounds like normal grammar-teaching Polish.