Televizor se ugasio usred filma.

Not literally. In this sentence, se is not best understood as English itself.

Here it helps turn the verb into an intransitive/change-of-state meaning:

• ugasiti = to turn something off
• ugasiti se = to turn off, go off, switch off

So:

• Ugasio sam televizor. = I turned off the TV.
• Televizor se ugasio. = The TV turned off / went off.

So se shows that the TV is the thing that ended up off, without naming an outside person who did it.

Ugasio is the masculine singular past participle of ugasiti se.

It agrees with the subject:

• televizor is masculine singular
• so the participle is ugasio

If the subject were feminine, you would get a feminine form:

• Lampa se ugasila. = The lamp went out.

If it were plural:

• Televizori su se ugasili. = The TVs turned off.

So the -o ending here matches a masculine singular subject.

This is a very common learner question. In this kind of 3rd-person singular se construction, Croatian normally uses the form without je:

• Televizor se ugasio
• Ona se probudila
• Vrata su se otvorila for plural still keeps su

So Televizor se ugasio is the natural form.

For learners, the safest practical rule is:

• in 3rd person singular with se, you will often see just se + participle
• in plural, the auxiliary is still there: su se

Because se is a clitic. Croatian clitics usually come near the beginning of the clause, typically in the second position.

So in a neutral sentence:

• Televizor se ugasio.

That is much more natural than putting se at the end.

You can think of se as a small unstressed word that likes to attach early in the sentence, rather than standing by itself wherever you want.

Because usred requires the genitive case.

The noun is:

• nominative: film
• genitive singular: filma

So:

• usred filma = in the middle of the movie

More examples with usred:

• usred noći = in the middle of the night
• usred razgovora = in the middle of a conversation
• usred sata = in the middle of class

So filma is there because of the preposition usred.

Usred means in the middle of.

In this sentence, it usually does not mean mathematically exactly at the 50% point. It more naturally means:

• during the movie
• right in the middle of the movie
• at an inconvenient moment while the movie was still going on

So Televizor se ugasio usred filma suggests the TV turned off at a bad moment while the movie was in progress.

This is a question about aspect.

• ugasiti se = perfective
• gasiti se = imperfective

In this sentence, Croatian uses ugasiti se because it describes one completed event: the TV turned off.

Compare:

• Televizor se ugasio. = The TV turned off.
  One completed event.

• Televizor se gasio cijelu večer. = The TV kept turning off / was turning off all evening.
  Repeated or ongoing situation.

So the sentence uses the perfective verb because it talks about a single finished incident.

Not with the same meaning.

Without se, ugasiti is normally transitive, so it usually needs an object:

• Ugasio sam televizor. = I turned off the TV.

If you said Televizor ugasio..., it would sound incomplete or wrong in this context, because it would suggest that the TV turned something else off.

So in this sentence, se is necessary to get the meaning the TV turned off rather than the TV turned something off.