Na spratu je naš stan, a lift nije dobar.

Spratu is in the locative singular.

That is because na means on/in/at here in a location sense, and with that meaning it takes the locative:

• na spratu = on the floor / upstairs

A very useful pattern is:

• na + locative for location
• na + accusative for motion toward something

For example:

• Stan je na spratu. = The apartment is on the floor / upstairs.
• Idem na sprat. = I’m going upstairs.

So in your sentence, na spratu is correct because it describes where the apartment is.

Because je is a clitic in Serbian. Clitics are short unstressed words that usually go in the second position of the clause.

In this sentence, the first unit is Na spratu, so je comes right after it:

• Na spratu je naš stan.

This is a very normal Serbian word order.

You can also say:

• Naš stan je na spratu.

That version may feel a bit more straightforward to an English speaker. The original sentence puts more focus first on the location: On the floor / upstairs is our apartment.

Because naš stan is the subject of the clause, so it stays in the nominative:

• naš = our
• stan = apartment

Both are masculine singular nominative here:

• naš stan = our apartment

The phrase na spratu is only telling you the location. It does not change the case of the subject.

So the structure is basically:

• Na spratu = location
• je = is
• naš stan = subject

A often means something like and, but it usually adds a sense of contrast, switch of topic, or whereas.

So:

• Na spratu je naš stan, a lift nije dobar.

means something like:

• Our apartment is upstairs, and/but the elevator isn’t good.

Here a works well because the second clause adds a contrasting or relevant fact: the apartment is upstairs, but the elevator is not good.

Compare:

• i = simple and
• a = and / but / whereas with a contrast or change of focus

Because the negative forms of biti (to be) are written as single words in the present tense:

• nisam = I am not
• nisi = you are not
• nije = he/she/it is not
• nismo = we are not
• niste = you are not
• nisu = they are not

So:

• lift nije dobar = the elevator is not good

You do not normally say ne je.

Because dobar must agree with lift.

Lift is a masculine singular noun, so the adjective also has to be masculine singular:

• dobar lift = a good elevator
• lift je dobar = the elevator is good

Compare the agreement:

• masculine: dobar
• feminine: dobra
• neuter: dobro

So dobro would not match lift.

Yes, lift is very common and completely normal.

You may also see dizalo, especially depending on region and style, but lift is widely used and very natural in everyday speech.

So:

• lift nije dobar sounds normal and idiomatic.

Usually sprat refers to a storey above the ground floor.

A useful distinction is:

• prizemlje = ground floor
• sprat = upper floor / storey

So na spratu often suggests upstairs or on an upper floor, unless the exact number is given.

For example:

• na prvom spratu = on the first floor
• na drugom spratu = on the second floor

This matters because English and Serbian do not always divide floors in exactly the same way.

Yes, absolutely.

That version is also correct:

• Naš stan je na spratu, a lift nije dobar.

The difference is mostly about focus and word order:

• Na spratu je naš stan... emphasizes the location first.
• Naš stan je na spratu... starts with the subject and may feel more neutral.

Both are grammatical and natural.

It can mean several things depending on context, including:

• good
• fine
• working well
• in good condition

In this sentence, lift nije dobar most naturally means something like:

• the elevator is not good
• the elevator is not in good condition
• the elevator isn’t working properly

In real conversation, Serbian often uses dobar in this broad everyday way.