Kupila sam novi sto, ali je stari još u hodniku.

In Serbian past tense, sam is a clitic, and clitics usually go in second position in the clause.

So in Kupila sam novi sto, the first element is kupila, and sam comes right after it.

This is the most neutral pattern here.
You may also hear Ja sam kupila... if the speaker wants to stress I, but without emphasis Serbian often leaves out the pronoun and places the clitic after the first word or phrase.

That ending shows the gender of the speaker in the past tense.

• kupila sam = a female speaker says I bought
• kupio sam = a male speaker says I bought

Serbian past tense is formed with:

• the l-participle: kupio / kupila / kupilo
• plus the auxiliary: sam

So kupila sam literally reflects bought + am, but it functions as the normal past tense: I bought.

Yes, the direct object is in the accusative, but for masculine inanimate nouns, the accusative is often the same as the nominative.

So:

• nominative: novi sto
• accusative: novi sto

That is why the form does not change here.

This happens because sto in this sentence means table, which is an inanimate masculine noun.

Yes, sto can mean both:

• table
• hundred

Here it clearly means table, because of the context:

• Kupila sam novi sto = I bought a new table
• stari još u hodniku = the old one is still in the hallway

So the sentence is about furniture, not numbers.

Again, this is because je is a clitic and clitics normally go in second position.

In the clause ali je stari još u hodniku:

• ali is the first element
• je comes right after it

So the structure is natural Serbian word order.

English speakers often expect something like but the old one is, but Serbian clitic placement works differently.

Yes. stari here stands for stari sto.

Serbian often leaves out a noun when it is already obvious from context. So:

• novi sto = new table
• stari = the old one / the old table

This is very natural. English does something similar with the old one.

Because the noun has been omitted, and the adjective is being used substantively, meaning it behaves like a noun.

So instead of repeating sto, Serbian just says stari.

In context:

• novi sto = a new table
• stari = the old one

Serbian does this quite often when the missing noun is easy to understand.

Here još means still.

So:

• još u hodniku = still in the hallway

Depending on context, još can also mean things like more, another, or yet, but in this sentence the correct meaning is still.

Because u can take different cases depending on meaning.

Here it means in a place, showing location, so it takes the locative case:

• nominative: hodnik
• locative: hodniku

So:

• u hodniku = in the hallway

If it were showing motion into the hallway, Serbian would use the accusative instead.

Serbian does not have articles like English a and the.

So whether something is understood as a new table or the new table depends on context.

In this sentence:

• novi sto is naturally understood as a new table
• stari is understood as the old one

The meaning comes from the situation, not from an article.

Because Serbian often drops subject pronouns when they are not needed.

In kupila sam, the verb form already tells you a lot, and the context tells you who is speaking, so ja is unnecessary.

• Kupila sam novi sto = I bought a new table
• Ja sam kupila novi sto would be possible, but it would usually add emphasis to I

This is very common in Serbian.

Ali means but.

It connects the two parts of the sentence:

• Kupila sam novi sto = I bought a new table
• ali je stari još u hodniku = but the old one is still in the hallway

So it shows contrast: a new table has been bought, but the old one has not been removed yet.