Gosti su ispred zgrade, a komšija otvara ulaz.

Because the two clauses are built differently.

• Gosti su ispred zgrade = The guests are in front of the building

  • Here su is the present-tense form of biti (to be) for they.
  • The clause needs a form of to be because ispred zgrade is not a conjugated verb phrase.

• komšija otvara ulaz = the neighbor opens / is opening the entrance

  • Here otvara is already a fully conjugated verb, so no extra is/am/are-type word is needed.

So:

• su = are
• otvara already means opens / is opening

Because ispred (in front of) requires the genitive case.

The base form is:

• zgrada = building

After ispred, Serbian uses genitive:

• ispred zgrade = in front of the building

This is very common in Serbian: many prepositions require a specific case, and ispred takes the genitive.

So:

• zgrada = dictionary form
• zgrade = genitive singular form used after ispred

Often yes, but not exactly.

In this sentence, a links two clauses and suggests a slight contrast, shift, or separate piece of information:

• Gosti su ispred zgrade, a komšija otvara ulaz.

A natural English translation may use and, but a often feels more like:

• and meanwhile
• while
• whereas
• and as for

It is different from:

• i = simple and
• ali = but

So a is a very common connector when Serbian moves from one subject or situation to another.

Because some masculine nouns in Serbian end in -a.

Komšija usually refers to a male neighbor, even though its ending looks like many feminine nouns. This is normal in Serbian. Another example is:

• sudija = judge (often masculine in reference)

So grammatical gender is not determined only by the final letter. You have to learn some nouns individually.

In this sentence:

• komšija is masculine
• it is the subject of otvara

It is in the accusative singular because it is the direct object of otvara.

• komšija otvara ulaz = the neighbor opens the entrance

With many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular. So:

• nominative: ulaz
• accusative: ulaz

That is why there is no visible ending change here.

It is present tense, from the imperfective verb otvarati (to open).

Depending on context, Serbian present tense can mean:

• opens
• is opening
• sometimes even a habitual action, like opens regularly

So in this sentence, otvara could be understood as:

• the neighbor opens the entrance
• the neighbor is opening the entrance

The exact English choice depends on context.

Because Serbian normally does not use articles.

So:

• gosti can mean guests or the guests
• komšija can mean a neighbor or the neighbor
• ulaz can mean an entrance or the entrance

The exact meaning is understood from context.

That is why one Serbian sentence can often be translated into English in more than one natural way.

Yes. Serbian word order is more flexible than English word order.

The basic version here is:

• Gosti su ispred zgrade, a komšija otvara ulaz.

But Serbian can rearrange words for emphasis, topic, or style. For example:

• Komšija otvara ulaz, a gosti su ispred zgrade.
• Gosti su ispred zgrade, a ulaz otvara komšija.

These versions are not all equally neutral, but they are possible. The original sentence sounds straightforward and natural.

Because a is connecting two separate clauses:

• Gosti su ispred zgrade
• komšija otvara ulaz

In Serbian, when clauses are joined by a, a comma is normally used. So the punctuation here is standard and expected.

They are important Serbian sounds:

• š sounds like sh in ship

  • komšija

• č sounds roughly like ch in chocolate

  • not used in this exact sentence, but useful to compare with other Serbian words

Also useful here:

• ž sounds like the s in measure
  • zgrade

So approximate pronunciation would be:

• Gosti su ispred zgrade, a komšija otvara ulaz.
• roughly: GO-stee soo ees-pred ZGRAH-deh, a KOM-shee-ya OT-va-ra OO-laz

It is plural.

The singular is:

• gost = guest

The plural is:

• gosti = guests

That is also why the sentence uses su:

• gosti su = the guests are

If it were singular, you would have:

• Gost je ispred zgrade. = The guest is in front of the building.