Moja sestra i ja sada idemo zajedno u grad.

Because moja has to agree with sestra.

In Serbian, possessive words like moj / moja / moje change according to the gender, number, and case of the noun they describe.

• sestra is feminine singular
• so the correct form is moja

Compare:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

So moja sestra is simply the feminine form needed here.

Because the speaker wants to say exactly my sister and I, not just we.

• mi = we
• moja sestra i ja = my sister and I

Both are possible, but they are not equally specific:

• Mi sada idemo zajedno u grad. = We are going to town now/together.
• Moja sestra i ja sada idemo zajedno u grad. = My sister and I are going to town together now.

Serbian often leaves pronouns out when the verb already shows the subject, but here the full phrase is used to name the people clearly.

Because the subject is two people: moja sestra i ja.

When two subjects are joined by i (and), the verb is plural. Since one of those people is ja (I), Serbian uses the 1st person plural form:

• idem = I go / I am going
• ideš = you go
• ide = he/she/it goes
• idemo = we go / we are going

So:

• Moja sestra ide = My sister goes / is going
• Moja sestra i ja idemo = My sister and I go / are going

No. It comes from ići (to go), which is an important irregular verb.

Some present-tense forms are:

• ja idem = I go
• ti ideš = you go
• on/ona ide = he/she goes
• mi idemo = we go
• vi idete = you go
• oni/one idu = they go

So idemo is the correct we form of ići.

It can mean both, depending on context.

Serbian present tense often covers both:

• we go
• we are going

So sada idemo most naturally means we are going now, because sada gives a clear right now / at this moment feeling.

Without sada, idemo could also be more general, like we go or we’re going depending on context.

Sada means now.

It tells you the action is happening at this moment or around now:

• Moja sestra i ja idemo u grad. = My sister and I are going to town / go to town.
• Moja sestra i ja sada idemo u grad. = My sister and I are going to town now.

In conversation, Serbian also often uses sad instead of sada:

• Moja sestra i ja sad idemo u grad.

That sounds very natural in everyday speech.

Zajedno means together.

It emphasizes that the speaker and the sister are going with each other, not separately.

So:

• Moja sestra i ja idemo u grad. = My sister and I are going to town.
• Moja sestra i ja idemo zajedno u grad. = My sister and I are going to town together.

It is an adverb, so it describes how they are going.

Because u can take different cases depending on whether there is movement or location.

Here there is movement toward a place:

• u grad = into town / to town

That uses the accusative case.

But for location, Serbian uses the locative:

• u gradu = in the city / in town

So the contrast is:

• Idemo u grad. = We are going to town.
• Mi smo u gradu. = We are in town.

This is a very important pattern in Serbian:

• u + accusative = motion toward/into
• u + locative = location in

Because Serbian has no articles like English a/an/the.

So grad can mean:

• a city
• the city
• town
• the town

The exact meaning depends on context.

In this sentence, u grad is often understood idiomatically as to town or into the city/town, depending on the situation.

English needs an article or another structure, but Serbian does not.

Yes, Serbian word order is relatively flexible.

The sentence:

• Moja sestra i ja sada idemo zajedno u grad.

is natural and clear. But other word orders are also possible, for example:

• Moja sestra i ja idemo sada zajedno u grad.
• Moja sestra i ja zajedno idemo u grad.
• Sada moja sestra i ja idemo zajedno u grad.

The basic meaning stays the same, but the focus can shift slightly.

For example:

• putting sada earlier can emphasize now
• putting zajedno earlier can emphasize together

So Serbian word order is not random, but it is more flexible than English.

Both are grammatically possible.

• moja sestra i ja
• ja i moja sestra

However, moja sestra i ja often sounds more polite or natural, because speakers commonly mention the other person before themselves.

This is similar to English, where my sister and I usually sounds more natural than I and my sister.

So the choice is mostly about style and politeness, not grammar.

Yes, if the context already makes it clear who we are.

Because idemo already shows we, Serbian often drops the subject:

• Sada idemo zajedno u grad. = We are going to town together now.

But if you want to specify exactly who we are, you keep the subject:

• Moja sestra i ja sada idemo zajedno u grad.

This is a very common feature of Serbian: subject pronouns and subject phrases are often omitted when they are already understood.

It can mean both city and town, depending on context.

So u grad may be translated as:

• to the city
• to town

In many everyday situations, especially from the perspective of someone outside the center, ići u grad is a very common expression meaning something like go into town.

So even if the literal word is city/town, the phrase often functions like an idiomatic everyday expression.

It is in the nominative case, because it is part of the subject.

The subject of the sentence is:

• Moja sestra i ja

That whole phrase is in nominative.

Compare:

• ja = I
• mene / me = me
• meni / mi = to me

Since the sentence means my sister and I, not my sister and me, nominative ja is required in Serbian as well.

Sometimes, yes.

Ići is the general verb to go, so idemo is perfectly correct for we are going.

But if you want to emphasize setting off or departure, Serbian may use verbs like:

• polazimo = we are setting off / departing
• krećemo = we are starting out / heading off

So:

• Moja sestra i ja sada idemo zajedno u grad. = My sister and I are going to town together now.
• Moja sestra i ja sada krećemo u grad. = My sister and I are heading to town now.

The original sentence is the most neutral and general way to say it.