Ten rzeczownik jest w liczbie mnogiej.

Because rzeczownik is a masculine singular noun in Polish. Demonstratives like ten / ta / to must agree with the noun’s gender, number, and case.

• ten = masculine singular
• ta = feminine singular
• to = neuter singular

So:

• ten rzeczownik = correct
• ta rzeczownik = incorrect
• to rzeczownik = incorrect in this sentence

It is in the nominative singular.

That is because rzeczownik is the subject of the sentence:

• Ten rzeczownik = the thing being talked about
• jest w liczbie mnogiej = is in the plural

So the basic structure is:

[subject] + [verb] + [complement]

Because the verb agrees with the subject, and the subject is ten rzeczownik, which is singular.

You are talking about one noun and describing it as being in plural form.

So the logic is:

• This noun = singular subject
• is = singular verb
• in the plural = description of its grammatical number

This is very similar to English: This noun is plural.
Even though plural is mentioned, this noun is still singular as the subject.

W liczbie mnogiej is a standard grammatical expression meaning in the plural.

It is the normal way to describe grammatical number in Polish:

• w liczbie mnogiej = in the plural
• w liczbie pojedynczej = in the singular

So Ten rzeczownik jest w liczbie mnogiej is a normal way to say that the noun is in a plural form.

Because the preposition w often requires the locative case when it means in.

The base form is:

• liczba = number

But after w, it changes to the locative:

• w liczbie = in the number

So:

• liczba mnoga = plural
• w liczbie mnogiej = in the plural

Because the adjective has to agree with liczbie.

Here, liczbie is:

• feminine
• singular
• locative

So the adjective must match that:

• dictionary form: mnoga
• in this sentence: mnogiej

This is normal Polish adjective agreement.

You can think of it like this:

• liczba mnoga = plural number
• w liczbie mnogiej = in plural number

Both words change as needed by the grammar.

The opposite is w liczbie pojedynczej, which means in the singular.

So you can compare:

• Ten rzeczownik jest w liczbie mnogiej. = This noun is in the plural.
• Ten rzeczownik jest w liczbie pojedynczej. = This noun is in the singular.

This pair is very common in grammar explanations.

You can, but it usually means something a little different.

• jest w liczbie mnogiej = is in the plural, is currently in plural form
• ma liczbę mnogą = has a plural form

So:

• Ten rzeczownik jest w liczbie mnogiej focuses on the form being used.
• Ten rzeczownik ma liczbę mnogą focuses on whether the noun has a plural at all.

For example, if you are analyzing a sentence and pointing to a word form, jest w liczbie mnogiej is the more direct choice.

Not necessarily.

It can simply mean that the noun is in a plural form here. It does not automatically mean that the noun exists only in the plural.

If you want to say that a noun exists only in plural form, Polish would usually say something more specific, such as:

Ten rzeczownik występuje tylko w liczbie mnogiej.

That means This noun occurs only in the plural.

So the original sentence is more general.

Yes, Polish word order is fairly flexible, but Ten rzeczownik jest w liczbie mnogiej is the most neutral and natural order.

This order is good because it clearly gives:

1. the topic: Ten rzeczownik
2. the verb: jest
3. the grammatical description: w liczbie mnogiej

Other orders are possible, but they usually add emphasis or sound less neutral. For a learner, the given version is the best default.