Das Büro ist im dritten Stock.

Büro is a neuter noun in German, so its basic article is das.

• das Büro = the office

In this sentence, das Büro is the subject, so it stays in the nominative case: Das Büro.

German noun gender is something you usually have to learn together with the noun:

• das Büro
• der Tisch
• die Tür

In German, all nouns are capitalized, not just ones at the beginning of a sentence.

So:

• das Büro
• der Stock
• die Stadt

This is one of the most noticeable spelling differences from English.

im is a contraction of in dem.

• in dem → im

So:

• im dritten Stock literally comes from in dem dritten Stock

This contraction is very common and sounds more natural than the full form in everyday German.

Because dritten is an adjective ending caused by the grammar around it.

Here, dritten is part of im dritten Stock = in dem dritten Stock.

Since:

• in is being used for location
• dem is dative singular
• dritte is describing Stock

the adjective takes the ending -en:

• in dem dritten Stock

So dritten is the dative form of the ordinal adjective dritte (third).

Because this sentence describes a location, not movement.

German in is a two-way preposition:

• dative for location: where something is
• accusative for direction/movement: where something is going

Here, the office is already located there:

• Das Büro ist im dritten Stock.
  = location → dative

Compare:

• Ich bin im Büro. = I am in the office
• Ich gehe ins Büro. = I am going into the office

Yes. dritten comes from dritte, the German ordinal number meaning third.

Some basic ordinals are:

• erste = first
• zweite = second
• dritte = third
• vierte = fourth

But ordinals change their endings depending on case, gender, and article. That is why you see dritten here instead of the base form dritte.

Yes. In this sentence, Stock means floor or storey of a building.

• der Stock
• im dritten Stock = on the third floor / on the third storey

Be careful: Stock can mean other things in other contexts too, such as a stick or cane, but here it clearly means a building level.

It depends on which kind of English you mean.

In German-speaking countries, the ground level is usually:

• das Erdgeschoss

Then the level above that is:

• der erste Stock

So:

• der dritte Stock is the third level above the ground floor

That matches British English more closely.

For American English, this can be confusing, because first floor usually means the ground level. So German dritter Stock may correspond to what an American speaker would call the fourth floor, depending on the building and context.

Yes, grammatically you can, because im simply means in dem.

But in normal German, im dritten Stock is much more natural and much more common.

So:

• im dritten Stock = normal, idiomatic
• in dem dritten Stock = correct, but less natural in everyday speech

This is a very standard German statement pattern:

• subject
  • verb
    • rest

So:

• Das Büro = subject
• ist = verb
• im dritten Stock = location

German main clauses usually keep the conjugated verb in second position. Here, the subject comes first, so the verb comes right after it.

You could also change the emphasis:

• Im dritten Stock ist das Büro.

That is still correct, because ist remains in the second position.

A helpful approximate pronunciation is:

• Büro ≈ byu-ROH

A few notes:

• ü does not sound exactly like English oo or you
• the stress is on the second syllable: bü-RO
• the final o is a clear vowel sound

If the ü is difficult, that is very normal for English speakers. It is one of the German vowel sounds that usually takes practice.

In standard German, you normally say:

• im dritten Stock
• or in der dritten Etage

Using auf here is generally not standard for this meaning.

So for building floors, im dritten Stock is the safe and natural choice.