Jedan metar kabla je potreban.

Because after a measurement noun like metar, Serbian normally puts the thing being measured in the genitive.

So:

• metar = meter
• kabla = of cable

This is the same pattern as:

• čaša vode = a glass of water
• kilogram šećera = a kilogram of sugar
• metar tkanine = a meter of fabric

The dictionary form is kabl, but after metar it becomes kabla.

Because this whole phrase is the subject of the sentence, so it is in the nominative case.

• jedan agrees with metar
• metar is masculine singular nominative
• so you get jedan metar

If the phrase had a different role in the sentence, it could change case. But here it is the thing that is needed, so nominative is correct.

It can often feel like either, depending on context.

In Serbian, jedan literally means one, but it can also work a bit like a/an when you want to introduce a single item. In this sentence, it most naturally means:

• one meter of cable if the exact quantity matters
• a meter of cable if the speaker is just stating the needed amount

So jedan may be strictly numerical, or just lightly emphasize that the amount is one.

Yes. You can also say:

• Metar kabla je potreban.

That still means a meter of cable is needed.

The version with jedan sounds a bit more explicit about the quantity one. Without jedan, the sentence is still completely natural.

Because it agrees with the subject, which is jedan metar kabla.

The head noun of that phrase is metar, and metar is:

• masculine
• singular

So the adjective must also be:

• masculine
• singular

That is why you get potreban.

It does not agree with kabla, because kabla is inside the of cable part of the phrase. The main noun is still metar.

Because je is a clitic form of biti and clitics in Serbian usually go in the second position of the clause.

Here, the first unit is:

• Jedan metar kabla

So je comes right after that:

• Jedan metar kabla je potreban.

This is very normal Serbian word order.

Yes, absolutely. That is also very natural.

Both are correct:

• Jedan metar kabla je potreban.
• Potreban je jedan metar kabla.

The difference is mostly focus:

• Jedan metar kabla je potreban puts the measured amount first
• Potreban je jedan metar kabla sounds a bit more like What is needed is one meter of cable

In everyday Serbian, the second version is often a little more natural.

It is grammatically singular.

Even though the phrase refers to an amount, the main noun is metar, and that noun is singular. That is why the verb and adjective are singular too:

• je = is
• potreban = needed, masculine singular

So Serbian treats one meter of cable as a singular unit.

The dictionary form is kabl.

Its forms include things like:

• nominative: kabl
• genitive: kabla
• dative: kablu

So kabla is just a declined form of kabl.

Yes. In everyday speech, people very often say:

• Treba jedan metar kabla.

This is shorter and very common in conversation.

The sentence Jedan metar kabla je potreban is correct and clear, but it sounds a bit more formal or structured.