Film počinje u sedam sati, ali se završava kasno.

Serbian does not have articles like the or a/an.

So film can mean the film, a film, or just film, depending on context. In this sentence, context tells you it means the film.

• Film počinje... = The film starts...

Because Serbian often uses the present tense for scheduled events, just like English does.

Compare English:

• The film starts at seven.
• The train leaves at six.

Serbian does the same:

• Film počinje u sedam sati.

So this is a normal way to talk about a timetable or fixed schedule.

Počinje is the present-tense form of the imperfective verb počinjati / počinjati se? No—here the relevant basic pair is:

• počinjati = to be starting / to start (imperfective)
• početi = to start, to begin (perfective)

In everyday Serbian, počinje is the natural form in a sentence like this because it describes a scheduled event.

• Film počinje u sedam sati. = natural
• Film će početi u sedam sati. = also possible, a bit more explicitly future

Počne is the present form of the perfective početi, but perfective present usually does not work like a normal present in this kind of sentence.

When Serbian gives a clock time, it commonly uses u plus the time expression:

• u jedan sat = at one o’clock
• u tri sata = at three o’clock
• u sedam sati = at seven o’clock

So here u corresponds to English at.

Literally, sedam sati is something like seven hours, but in this context it simply means seven o’clock.

Because Serbian changes the form of sat depending on the number:

• jedan sat = one hour
• dva / tri / četiri sata = two / three / four hours
• pet, šest, sedam... sati = five, six, seven... hours

So:

• sedam sati is correct
• not sedam sat
• not sedam sata

This number pattern is very important in Serbian.

Yes, very often.

In everyday speech, u sedam is extremely common and natural:

• Film počinje u sedam.

Adding sati is a little fuller and slightly more explicit, but both are correct.

Here se završava means ends or comes to an end.

The little word se is a reflexive clitic, but in Serbian it does much more than literal reflexive meanings like wash oneself. With many verbs, it helps create an intransitive, middle, or passive-like meaning.

So:

• završavati can mean to finish something
• završavati se means to finish / to end

In this sentence:

• Film se završava kasno. = The film ends late.

It does not mean the film is literally doing something to itself in the English sense.

Because se is a clitic, and Serbian clitics usually go very early in the clause, often in second position.

So:

• ali se završava kasno = natural
• ali završava se kasno = not standard in this neutral sentence

This word order is something learners usually need to get used to: short words like se, je, sam, ga, mu often have fixed positions.

Kasno is an adverb here, meaning late. It describes the verb završava se.

• završava se kasno = ends late

Compare:

• kasan = late (masculine adjective)
• kasna = late (feminine adjective)
• kasno = late (neuter adjective) or late (adverb)

In this sentence, it is functioning as an adverb, not an adjective.

Yes, Serbian can sometimes use either ali or a, but they are not exactly the same.

• ali = but, stronger contrast
• a = and / while / but, often softer or more neutral contrast

Here ali emphasizes the contrast:

• it starts at seven, but it ends late

So ali is a very natural choice.

Because Serbian normally puts a comma before ali when it joins two clauses.

So:

• Film počinje u sedam sati, ali se završava kasno.

This is standard punctuation, just like English often uses a comma before but when connecting two full clauses.

The most important ones here are:

• č = like ch in church
• š = like sh in shoe
• ž = like the s in measure
• j = like y in yes
• nj = like ny in canyon or Spanish ñ

So roughly:

• počinje ≈ PO-chee-nye
• završava ≈ za-VR-sha-va

That is only an approximation, but it helps.

Film is in the nominative case because it is the subject of the sentence.

It is the thing that:

• počinje = starts
• se završava = ends

So the basic structure is:

• Film = subject
• počinje = verb
• u sedam sati = time expression
• ali se završava kasno = contrasting second clause