Ako ekran sutra ne bude radio, odneću telefon u prodavnicu.

It’s not past tense here. Ne bude radio is future II (also called futur II in Serbian).

In this sentence:

• bude = a form of biti
• radio = the active participle of raditi (to work / function)

Together, bude radio means something like will be working / will work in a conditional or time-dependent future context.

So:

• Ako ekran sutra ne bude radio... = If the screen isn’t working tomorrow...

Even though radio looks like a past form, in this structure it is part of a future construction.

Because after ako when talking about a future condition, Serbian normally uses future II, not future I.

So the standard pattern is:

• Ako + future II, future I

That gives:

• Ako ekran sutra ne bude radio, odneću telefon u prodavnicu.

This is the normal Serbian equivalent of English If the screen doesn’t work tomorrow, I’ll take the phone to the store.

Using neće raditi after ako is generally not the standard choice in this kind of sentence.

Odneću is future I, first person singular: I will take.

It comes from the verb odneti (to take away / take somewhere).

So:

• odneću = I will take

This is the main action that will happen if the condition is met.

Because Serbian usually leaves out subject pronouns when they are already clear from the verb form.

In odneću, the ending shows that the subject is I.

So Serbian does not need ja here:

• Odneću telefon... = I’ll take the phone...

You could say ja ću odneti telefon or ja ću ga odneti, but ja would usually add emphasis, like I will.

Because odneti is perfective, and that fits a single completed future action.

Here the speaker means one concrete action:

• If the screen doesn’t work, I’ll take the phone to the store.

That is a one-time event, so odneti is natural.

By contrast:

• nositi is imperfective
• it often suggests an ongoing, repeated, or general action

So odneću sounds more natural than nosiću in this sentence.

Ekran is in the nominative singular because it is the subject of the clause.

The clause is:

• ekran ... ne bude radio = the screen ... doesn’t work

So ekran is the thing doing the action of working / functioning.

Also, Serbian has no articles, so ekran can mean:

• the screen
• a screen

The context tells you which one is meant.

It is in the accusative, but it looks the same as the nominative because telefon is a masculine inanimate noun.

In Serbian, many masculine inanimate nouns have:

• nominative: telefon
• accusative: telefon

So in:

• odneću telefon = I’ll take the phone

the word is accusative, even though it looks unchanged.

Because Serbian uses different cases depending on whether you mean movement or location.

• u + accusative = motion into / to
• u + locative = being in / at

So:

• u prodavnicu = to the store / into the store
• u prodavnici = in the store

Since the phone is being taken somewhere, u prodavnicu is correct.

Here, radio comes from the verb raditi and means worked / functioning as part of the verb phrase bude radio.

It is not the noun radio meaning the device.

So in this sentence:

• ekran ne bude radio = the screen doesn’t work / isn’t functioning

It just happens that the written form is the same as the noun radio.

Yes. Serbian word order is fairly flexible.

For example, you could also say:

• Ako sutra ekran ne bude radio, odneću telefon u prodavnicu.
• Odneću telefon u prodavnicu ako ekran sutra ne bude radio.

These all mean basically the same thing. The difference is mostly about emphasis and style.

The original version is natural and clear.

Because the sentence begins with a conditional clause:

• Ako ekran sutra ne bude radio = If the screen doesn’t work tomorrow

and then comes the main clause:

• odneću telefon u prodavnicu = I’ll take the phone to the store

In Serbian, when this kind of dependent clause comes first, a comma is used before the main clause.

So the comma helps separate:

• the condition
• from the result