Zaštitićemo fotelju ćebetom da bismo je preneli stepeništem.

Because this is a standard way of forming the future tense in Serbian.

With many verbs, you can say the future in two common ways:

• Zaštitićemo fotelju
• Ćemo zaštititi fotelju

Both mean the same thing.

What happens in zaštitićemo is:

• infinitive: zaštititi
• future auxiliary: ćemo
• when the infinitive comes first, the ending -ti drops and the auxiliary attaches

So:

• zaštititi + ćemo → zaštitićemo

This is very common in Serbian:

• uraditi → uradićemo
• poneti → ponećemo
• videti → videćemo

Because fotelju is the accusative singular form, and here fotelja is the direct object of zaštitićemo.

Cases here:

• nominative: fotelja = the armchair
• accusative: fotelju = the armchair, as the thing being acted on

So:

• Fotelja je stara. = The armchair is old.
• Zaštitićemo fotelju. = We will protect the armchair.

For many feminine nouns ending in -a, the accusative singular ends in -u.

Because ćebetom is instrumental singular, and Serbian uses the instrumental here to express the means or material used to do something.

The pattern is:

• zaštititi nešto nečim = to protect something with something

So:

• ćebe = blanket
• ćebetom = with a blanket

This is very natural Serbian grammar, similar to:

• seći nožem = cut with a knife
• pisati olovkom = write with a pencil
• pokriti nečim = cover with something

Da bismo introduces a purpose clause. It means something like so that or in order to.

So the structure is:

• Zaštitićemo fotelju ćebetom = main action
• da bismo je preneli stepeništem = purpose: why we are doing it

Literally, Serbian often uses:

• da + conditional/potential form

So da bismo is a very common way to say in order for us to... / so that we can...

Examples:

• Došao sam ranije da bismo razgovarali.
• Uzećemo taksi da bismo stigli na vreme.

Because Serbian usually does not use an English-style bare infinitive after in order to.

English says:

• in order to carry it

Serbian normally says:

• da bismo je preneli

This is one of the most important differences from English. Very often, where English uses an infinitive, Serbian prefers a da-clause.

So instead of something like:

• ... za preneti or similar

standard Serbian uses:

• ... da bismo preneli

Because preneli is the participle used with bismo in the conditional/potential construction.

The full pattern is:

• bih preneo / prenela
• bi preneo / prenela
• bismo preneli / prenele
• biste preneli / prenele
• bi preneli / prenele

In your sentence, the subject is we, so the form is bismo preneli.

Also, preneli is masculine plural. In Serbian, that is the default plural form for a mixed group or an unspecified group. If the speakers were all female, you could get:

• da bismo je prenele

Je refers back to fotelju.

Here it is the short accusative form of ona:

• je = her / it

Because fotelja is a feminine noun, the pronoun used for it is feminine.

So:

• da bismo je preneli = so that we could carry it

Serbian often uses a pronoun instead of repeating the noun. Repeating fotelju would be possible, but the pronoun is more natural here.

Because je is a clitic, and Serbian clitics have special word-order rules.

Short unstressed forms like:

• je
• ga
• mi
• ti
• se
• ću, ćeš, će, ćemo...
• bih, bi, bismo...

tend to stand early in the clause, in a fixed clitic position.

So in:

• da bismo je preneli

the order is natural because both bismo and je are clitics, and they appear before the main verb preneli.

An English speaker often expects freer placement of pronouns, but in Serbian these short forms are much more position-sensitive.

Because Serbian can use the instrumental case to express the route, path, or means of movement.

So:

• stepenište = staircase / stairwell
• stepeništem = through/by way of the staircase

This is similar to other Serbian expressions such as:

• ići ulicom = go along the street
• proći hodnikom = go through the hallway
• sići liftom = go down by elevator

So preneli stepeništem means carrying it via the staircase, along the staircase, or through the stairwell.

Both are possible in Serbian, but they are not exactly the same noun.

• stepenište = staircase, stairwell
• stepenice = stairs, steps

So:

• stepeništem focuses more on the staircase/stairwell as the route
• stepenicama would focus more on the stairs themselves

In real life, both kinds of expressions can appear depending on context. For an English speaker, it is useful to notice that Serbian may use a singular noun where English often uses plural stairs.

Because the sentence talks about complete, goal-oriented actions.

• zaštititi = to protect fully / successfully
• preneti = to carry/move over successfully from one place to another

The idea is:

1. we will protect the armchair
2. then we will carry it through the staircase

Both are seen as single completed actions with a clear result.

If you used imperfective verbs, the meaning would shift more toward an ongoing process, repetition, or general activity.

For example:

• štititi = be protecting / protect in general
• prenositi = be carrying/transferring repeatedly or as a process

So the perfective forms fit this one-time practical situation very well.

Yes, Serbian word order is fairly flexible, but not completely free.

You could also hear things like:

• Fotelju ćemo zaštititi ćebetom da bismo je preneli stepeništem.
• Ćebetom ćemo zaštititi fotelju da bismo je preneli stepeništem.

These versions shift emphasis a bit:

• starting with fotelju emphasizes the object
• starting with ćebetom emphasizes the means

However, the clitics still have to obey their usual rules, so you cannot move ćemo, bismo, and je completely at random.

The original sentence sounds natural and neutral.