Premiéra začala, sotva jsme koupili popcorn.

Because premiéra is a feminine noun in Czech.

In the past tense, the Czech verb agrees with the subject in gender and number. So:

• premiéra začala = the premiere began
• if the subject were masculine, you would get something like film začal
• if it were neuter, představení začalo

So začala tells you that the subject is feminine singular.

This is about Czech word order and the position of short auxiliary forms.

In the clause sotva jsme koupili popcorn, the word jsme is a clitic, and clitics usually go in the second position in the clause.

Here the first element is sotva, so jsme comes right after it:

• sotva jsme koupili popcorn

This is the natural order.

You can very often say koupili jsme in other contexts, for example when something else stands first in the clause:

• Popcorn jsme koupili pozdě.

But after sotva, jsme normally comes immediately after it.

In this sentence, sotva means something like:

• hardly / scarcely
• barely
• or more naturally in English here: no sooner than / as soon as

So the sentence suggests that the premiere started almost immediately after they bought the popcorn.

It often gives a feeling of very little time passing between the two actions.

Compare the nuance:

• jakmile = as soon as
• sotva = hardly had... when / no sooner had... than

So sotva is not just about sequence; it also suggests immediacy.

Because sotva jsme koupili popcorn is a subordinate clause.

In standard Czech spelling, subordinate clauses are separated by commas. Since sotva introduces that clause, you write:

• Premiéra začala, sotva jsme koupili popcorn.

If you reverse the order, you also keep the comma:

• Sotva jsme koupili popcorn, premiéra začala.

Because the subject is we, even though Czech does not have to state my explicitly.

The form jsme already shows 1st person plural = we.
So koupili jsme means we bought.

In the past tense, Czech uses:

• an auxiliary: jsem, jsi, jsme, jste
• plus a past participle: koupil / koupila / koupili / koupily, etc.

So jsme koupili = we bought

A useful extra detail:
koupili is the plural form used for a mixed group or a group including at least one male. If the speakers were all female, it would be:

• sotva jsme koupily popcorn

Here popcorn is in the accusative, because it is the direct object of koupili.

But for masculine inanimate nouns, the accusative singular is often the same as the nominative singular. So:

• ten popcorn = nominative
• koupili popcorn = accusative

So it does have the correct case; it just happens to look the same.

You may still see other forms in other cases, for example:

• bez popcornu = without popcorn
• k popcornu = to/with the popcorn

Because the sentence describes two completed events:

• the buying of the popcorn was completed
• then the premiere began

Perfective verbs are very common when Czech talks about a sequence of single completed actions.

Here:

• začala comes from začít = to begin, a perfective verb
• koupili comes from koupit = to buy, also perfective

That fits the meaning very well: one action was completed, and almost immediately the next one happened.

If you used imperfective forms, the meaning would change:

• kupovali jsme popcorn = we were buying popcorn / used to buy popcorn
• premiéra začínala = the premiere was beginning / used to begin

That would sound less like a sharp, completed sequence.

Yes, absolutely.

That version is very natural, and many learners may find it easier because the time clause comes first:

• Sotva jsme koupili popcorn, premiéra začala.

Both versions are correct:

• Premiéra začala, sotva jsme koupili popcorn.
• Sotva jsme koupili popcorn, premiéra začala.

The difference is mostly about emphasis and style, not basic meaning.

No. Premiéra means a premiere, so it can be the first public performance or showing of many things, for example:

• a film
• a play
• an opera
• another stage production

So the word is broader than just cinema.

Not always literally.

Word-for-word, sotva often matches hardly / scarcely / barely. But in sentences like this one, a more natural English translation is often:

• No sooner had we bought popcorn than the premiere began
• The premiere started as soon as we bought popcorn
• The premiere started right after we bought popcorn

So when learning it, it is better to focus on the function in the sentence, not just one dictionary equivalent.

It would be similar, but not exactly the same in tone.

• jakmile = as soon as
• sotva = hardly had... when / no sooner had... than

So:

• Jakmile jsme koupili popcorn, premiéra začala.
  = As soon as we bought popcorn, the premiere began.

• Sotva jsme koupili popcorn, premiéra začala.
  = We had barely bought popcorn when the premiere began.

The second one feels a bit stronger and emphasizes how immediate or even slightly annoyingly fast the next event was.