W liczbie mnogiej te rzeczowniki brzmią inaczej niż w liczbie pojedynczej.

Literally, it means in the plural number. In natural English, we usually say in the plural.

So:

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

This is a very standard way to talk about grammatical number in Polish.

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

The basic dictionary form is:

• liczba = number

But after w, it becomes:

• w liczbie = in the number

That is why the adjectives also change form:

• mnoga → mnogiej
• pojedyncza → pojedynczej

They have to agree with liczbie in case, gender, and number.

They are modifying liczbie, so they must match it.

Since liczba is:

• feminine
• singular
• here in the locative

the adjectives must also be:

• feminine
• singular
• locative

So:

• w liczbie mnogiej
• w liczbie pojedynczej

This is just normal adjective agreement in Polish.

Because rzeczowniki here are non-masculine-personal plural.

In Polish plural, demonstratives and adjectives often have different forms depending on whether the group is:

• masculine personal (usually groups of men / male people)
• non-masculine-personal (things, animals, mixed categories in many contexts, words, etc.)

So:

• ci = for masculine-personal plural
• te = for non-masculine-personal plural

Even though rzeczownik is a masculine noun in the singular, the plural rzeczowniki refers to nouns/words, not male people, so it takes te.

Because the sentence is talking about these nouns as a group.

Singular:

• rzeczownik = noun

Plural:

• rzeczowniki = nouns

Since te rzeczowniki is the subject of the sentence, the verb also has to be plural:

• te rzeczowniki brzmią

Brzmią is the 3rd person plural form of brzmieć (to sound).

Because the subject is plural:

• te rzeczowniki = these nouns

the verb must also be plural:

• brzmią = sound

Compare:

• ten rzeczownik brzmi inaczej = this noun sounds different
• te rzeczowniki brzmią inaczej = these nouns sound different

Yes. Brzmieć means to sound, and it is commonly used when talking about how words, names, sentences, or sounds are heard.

So in this sentence, it means that the nouns sound different in the plural form than in the singular form.

This is very natural Polish.

Because inaczej is an adverb, and it modifies the verb brzmią.

The sentence is describing how they sound, so Polish uses an adverb:

• brzmią inaczej = they sound differently / they sound different

If you used inne, that would be an adjective meaning different/other, but it would describe the nouns themselves, not the way they sound.

Niż means than in comparisons.

So:

• inaczej niż = differently than / more naturally, different from how ...

In the sentence:

• brzmią inaczej niż w liczbie pojedynczej

the idea is:

• they sound different than they do in the singular

This is a very common comparison structure in Polish.

In this sentence, the important case marker is really w, not niż.

The phrase is:

• w liczbie pojedynczej

and w requires the locative, which is why you get:

• liczbie
• pojedynczej

So the case here is determined by the preposition w inside that phrase.

No, Polish word order is fairly flexible.

The original sentence:

• W liczbie mnogiej te rzeczowniki brzmią inaczej niż w liczbie pojedynczej.

is natural and emphasizes the condition in the plural first.

You could also say:

• Te rzeczowniki w liczbie mnogiej brzmią inaczej niż w liczbie pojedynczej.

That is also correct, but the emphasis is a little different. The original version sounds slightly more explanatory or textbook-like.

Yes, very much so.

In grammar explanations, these are the standard expressions:

• liczba pojedyncza = singular
• liczba mnoga = plural

And when used with w:

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

You will see these expressions often in dictionaries, grammar books, and language lessons.

A more literal version would be:

• In the plural, these nouns sound differently than in the singular.

That is a bit awkward in English, so a more natural English version is usually:

• These nouns sound different in the plural than in the singular.

or

• In the plural, these nouns sound different from how they do in the singular.

Brzmią can be tricky for English speakers.

A rough guide is:

• brz sounds like the beginning of measure plus an r
• mią sounds roughly like myon with a nasal vowel at the end

A very rough approximation is:

• bzhmyon

But the nasal vowel ą does not have an exact English equivalent, so it is best learned by listening to native speech.

Sometimes, yes, if the context is very clear.

For example, you might hear something shorter like:

• W liczbie mnogiej te rzeczowniki brzmią inaczej niż w pojedynczej.

But the full version:

• niż w liczbie pojedynczej

is clearer and more standard, especially in teaching or formal explanation.