Komšinica će doći na rođendan sa cvećem.

Komšinica is a feminine singular noun in the nominative case, so it is the subject of the sentence. It specifically means a female neighbor.

• komšinica = female neighbor
• komšija = male neighbor

Because Serbian marks grammatical relationships with case endings, the nominative here shows that this is the person doing the action.

Serbian does not have articles like English the and a/an. So komšinica can mean either the neighbor or a neighbor, depending on context.

The same applies to other nouns in the sentence:

• na rođendan could be to the birthday party or to a birthday party
• sa cvećem could be with flowers

English has to choose an article; Serbian usually does not.

Će is the short form of the auxiliary verb used to make the future tense.
So:

• doći = to come
• će doći = will come

In Serbian, short forms like će are enclitics, which usually go in second position in the sentence or clause. That is why you get:

• Komšinica će doći...

and not normally a fully free English-style placement.

You can also change the sentence for emphasis, for example:

• Doći će komšinica...

but the clitic still stays near the beginning.

Yes. In modern Serbian, especially in everyday speech in Serbia, you will often hear:

• Komšinica će da dođe

This means the same as:

• Komšinica će doći

A simple way to think about it:

• će + infinitive = standard, very common, often taught first
• će da + present = also very common in speech

So both are normal, but your sentence uses the infinitive pattern.

This is a question of aspect, which is very important in Serbian.

• doći = perfective
  Focuses on one completed arrival
• dolaziti = imperfective
  Focuses on repeated, ongoing, or habitual coming

In this sentence, the idea is one future event: the neighbor will come once, to that birthday occasion. So doći is the natural choice.

Compare:

• Komšinica će doći na rođendan. = The neighbor will come to the birthday party.
• Komšinica će dolaziti... = The neighbor will be coming / will come repeatedly

Because this sentence describes movement toward an event, not being already there.

• na rođendan = to the birthday party / birthday celebration
• na rođendanu = at the birthday party

So:

• Komšinica će doći na rođendan = She will come to the birthday party.
• Komšinica je na rođendanu = She is at the birthday party.

Grammatically:

• na + accusative often marks motion toward an event
• na + locative often marks location at an event

Also, rođendan is a masculine inanimate noun, so its accusative singular looks the same as its nominative singular. That is why you see rođendan, not a visibly different ending.

Because the preposition sa / s meaning with requires the instrumental case.

The base noun is:

• cveće = flowers

After sa, it changes to instrumental:

• sa cvećem = with flowers

So the case ending is there because of the preposition.

Because cveće is a collective noun in Serbian. It is grammatically singular, but in meaning it corresponds to English flowers in a general sense.

So:

• cveće = flowers
• cvećem = with flowers

Even though English uses a plural noun, Serbian uses this collective singular form.

If you meant with one flower, you would use:

• sa cvetom = with a flower

S and sa are two forms of the same preposition meaning with.

Sa is often used when pronunciation is easier that way, especially before certain consonants or consonant clusters. In this sentence, sa cvećem sounds smoother than s cvećem for many speakers.

So:

• sa cvećem = very natural
• s cvećem = possible in some usage, but less smooth here

A learner can safely remember that sa is often chosen for ease of pronunciation.

Yes. Serbian word order is fairly flexible, because cases help show who is doing what. Changing the order usually changes emphasis, not the core meaning.

For example, these are all possible with different focus:

• Komšinica će doći na rođendan sa cvećem.
• Na rođendan će doći komšinica sa cvećem.
• Sa cvećem će komšinica doći na rođendan.

The basic idea stays the same, but the speaker may be highlighting:

• who is coming
• where she is coming
• what she is bringing

The important thing to watch is that the clitic će still behaves like a second-position element.

Yes. Serbian is commonly written in both Latin and Cyrillic scripts.

The Cyrillic version is:

Комшиница ће доћи на рођендан са цвећем.

It means exactly the same thing as the Latin-script version.