Opěrka zad je na kancelářské židli.

Opěrka means support or rest. In furniture vocabulary, opěrka zad means backrest.

Literally, it is something like support of the back:

• opěrka = support/rest
• zad = of the back

So opěrka zad is a set phrase for backrest.

Because záda is the basic dictionary form, but here Czech needs a different case.

The noun záda:

• means back in the body sense
• is normally used only in the plural

In opěrka zad, the word zad is in the genitive plural, because the phrase means backrest of the back or more naturally back support.

So:

• záda = back
• opěrka zad = backrest

This is a very common Czech pattern: one noun followed by another noun in the genitive.

Je is the 3rd person singular form of the verb být (to be).

So:

• je = is

The sentence structure is:

• Opěrka zad = the backrest
• je = is
• na kancelářské židli = on the office chair

So the full sentence literally means: The backrest is on the office chair.

Czech does not have articles like English a/an and the.

So:

• opěrka can mean a backrest or the backrest
• kancelářská židle can mean an office chair or the office chair

You understand which one is meant from the context.

That is completely normal in Czech.

Because the preposition na can mean on, and when it expresses location, it takes the locative case.

Here the idea is location:

• the backrest is on the chair

So:

• na
  • locative

That is why kancelářská židle changes to:

• na kancelářské židli

Because Czech adjectives must agree with the noun they describe in:

• gender
• number
• case

The noun is židle:

• feminine
• singular

After na expressing location, it must be in the locative singular:

• židle → židli

The adjective kancelářská must match that:

• kancelářská → kancelářské

So:

• nominative: kancelářská židle
• locative after na: na kancelářské židli

The whole subject opěrka zad is in the nominative, because it is the thing that is somewhere.

Inside that phrase:

• opěrka is nominative singular
• zad is genitive plural

So the grammar is:

• Opěrka = nominative subject
• zad = genitive dependent noun

No. Na can take different cases depending on meaning.

1. Location → locative

   • na židli = on the chair

2. Direction toward something → accusative

   • na židli can also be accusative in some forms, but with adjectives you can see the difference more clearly:
   • na kancelářskou židli = onto the office chair

So compare:

• je na kancelářské židli = it is on the office chair
• dává to na kancelářskou židli = he/she puts it onto the office chair

Židle is the dictionary form, but Czech nouns change according to case.

For this noun:

• nominative singular: židle
• locative singular: židli

Since na here expresses location, Czech uses the locative:

• na židli
• na kancelářské židli

This kind of change is one of the basic features of Czech grammar.

Both are feminine singular nouns.

You can see this from their dictionary forms:

• židle
• opěrka

That is also why the adjective appears as kancelářská in the basic form:

• kancelářská židle

Yes. Czech word order is more flexible than English.

This sentence could also be:

• Na kancelářské židli je opěrka zad.

Both versions are grammatical, but the emphasis changes slightly.

• Opěrka zad je na kancelářské židli.
  Focuses first on the backrest.

• Na kancelářské židli je opěrka zad.
  Focuses first on the location.

In neutral explanation or description, the original sentence is perfectly fine.

It is grammatical and understandable, but depending on context, a Czech speaker might also say:

• Kancelářská židle má opěrku zad. = The office chair has a backrest.

That version sounds more natural if the point is describing a feature of the chair.

The original sentence Opěrka zad je na kancelářské židli is more literal: it states where the backrest is.

So the best sentence depends on what you want to emphasize:

• location → Opěrka zad je na kancelářské židli.
• feature/possession → Kancelářská židle má opěrku zad.

The noun kancelář means office.

The adjective kancelářský / kancelářská / kancelářské means:

• office
• for office use
• related to an office

So:

• kancelářská židle = office chair

In the sentence, the adjective changes form because it must agree with židli:

• basic form: kancelářská
• after na with locative: kancelářské

Usually from context.

In English, The backrest is on the office chair can sound a bit physical, as if something is placed on top of something else. In Czech, na židli is also the normal prepositional phrase for being on a chair, and it can work in descriptions of parts.

But if you want to clearly say that the chair has a backrest as one of its features, Czech often prefers:

• Kancelářská židle má opěrku zad.

So:

• je na = is on
• má = has

The original sentence is fine, but má opěrku zad is often more natural for describing the chair’s design.