Kada idem u grad, nosim torbu sa papirom i olovkom.

In Serbian, subject pronouns are often omitted because the verb ending already shows who is doing the action.

• idem = I go / I am going
• nosim = I carry / I wear / I take

So Kada idem u grad, nosim torbu... already clearly means When I go into town, I carry a bag...
You could add ja for emphasis, but it is not necessary in a neutral sentence.

Kada means when and introduces a time clause.

So:

• Kada idem u grad = When I go into town

It connects the first part of the sentence with the main clause:

• Kada idem u grad, nosim torbu...

In everyday speech, kad is also very common and means the same thing here.
So Kad idem u grad... would sound natural too.

Serbian uses the present tense for habitual actions, just like English often does.

So this sentence does not mean only right now. It can mean something like:

• Whenever I go into town, I carry a bag with paper and a pencil
• When I go into town, I take a bag with paper and a pencil

This is a normal use of the present tense in Serbian.

Because u can take different cases depending on meaning:

• u + accusative = movement into/to a place
• u + locative = being in a place

Here there is movement:

• idem u grad = I go to / into town

Compare:

• Ja sam u gradu = I am in town
• Idem u grad = I am going to town

So grad is in the accusative here.

Not always. In this expression, ići u grad often means to go into town, to go to the city center, or to go out into the urban area, depending on context.

So even if the basic meaning of grad is city/town, the phrase idem u grad is a very common expression and may sound more natural than a literal English I go into the city in some contexts.

Because torbu is the accusative singular form of torba.

The verb nositi takes a direct object, and direct objects are often in the accusative case.

• nominative: torba = bag
• accusative: torbu = bag as the object of the verb

So:

• nosim torbu = I carry a bag

This is especially easy to notice with many feminine nouns ending in -a:

• knjiga → knjigu
• olovka → olovku
• torba → torbu

Because after sa in this sentence, Serbian uses the instrumental case.

• sa papirom = with paper
• sa olovkom = with a pencil

The instrumental endings here are:

• papir → papirom
• olovka → olovkom

So sa papirom i olovkom means with paper and a pencil.

Both s and sa mean with.
Sa is often used for easier pronunciation, especially before certain consonant clusters or sounds that would be awkward with just s.

In modern standard Serbian:

• s tobom
• sa mnom
• sa papirom

So sa papirom is the normal form here.
You should learn sa as the safe choice in phrases like this.

Yes, Serbian could use plurals in some contexts, but the singular here is perfectly natural.

• sa papirom i olovkom = with paper and a pencil

Here papir is being used as a material or general item, much like English paper.
And olovkom in the singular suggests one pencil.

If you wanted plural, it would change the meaning:

• sa papirima i olovkama = with papers and pencils

That sounds more like several separate items.

Because Kada idem u grad is a subordinate clause placed before the main clause.

Structure:

• subordinate clause: Kada idem u grad
• main clause: nosim torbu sa papirom i olovkom

In Serbian, it is normal to separate that introductory clause with a comma.

Yes. Serbian word order is fairly flexible, although some versions sound more neutral than others.

The original sentence:

• Kada idem u grad, nosim torbu sa papirom i olovkom.

You could also say:

• Nosim torbu sa papirom i olovkom kada idem u grad.

That still makes sense, but the original version feels more natural if you want to emphasize the when part first.

Serbian often uses word order for focus and style rather than strict grammatical necessity.

Not exactly. Nositi is a broad verb and can mean things like:

• carry
• wear
• take along

In this sentence, nosim torbu is best understood as I carry a bag or I take a bag with me.

So the exact English translation can vary depending on context, but the Serbian is completely natural.

Grammatically, it most naturally describes the bag:

• nosim torbu sa papirom i olovkom = I carry a bag with paper and a pencil

So the idea is that the bag contains those things.

If you wanted to emphasize that you have those items with you, Serbian could also express that differently, but in this sentence the default reading is that the bag is the thing with paper and a pencil.