Den film var den mest interessante del af aftenen, selv om emnet først virkede meget kompliceret.

Filmen is the ordinary definite form meaning the film.
Den film is usually more demonstrative, closer to that film or that particular film.

So this wording suggests the speaker is singling out one film in particular, perhaps in contrast with other parts of the evening.

A useful shortcut:

• filmen = the film
• den film = that film / that particular film

This is the normal way to form a definite superlative phrase in Danish:

• den = the for a common-gender noun
• mest interessante = most interesting
• del = part

So:

• den mest interessante del = the most interesting part

Two important things are happening here:

1. interessant is a longer adjective, so Danish normally compares it with mere and mest:

   • interessant = interesting
   • mere interessant = more interesting
   • mest interessant = most interesting

2. In a definite phrase like this, the adjective takes -e:

   • den mest interessante del

Also, del is a common-gender noun, so the article is den, not det.

Because the adjective is part of a definite noun phrase.

In Danish, adjectives usually take -e when they stand before a noun in a definite phrase:

• en interessant del = an interesting part
• den interessante del = the interesting part

The same happens in superlatives:

• den mest interessante del = the most interesting part

So the -e here is not random; it is a normal agreement pattern for adjectives in definite phrases.

Because Danish uses del af ... for part of ....

So:

• en del af aftenen = a part of the evening
• den mest interessante del af aftenen = the most interesting part of the evening

Here af means of.

This is a very common pattern in Danish:

• slutningen af filmen = the end of the film
• midten af bogen = the middle of the book

Here selv om means although / even though.

It introduces a subordinate clause:

• selv om emnet først virkede meget kompliceret
• even though the subject at first seemed very complicated

You will also often see selvom written as one word in modern Danish. In practice, learners should recognize both spellings when reading, but the meaning here is the same: although / even though.

Because selv om introduces a subordinate clause, and Danish subordinate clauses do not follow the same word-order rules as main clauses.

In a main clause, Danish normally follows the V2 pattern, where the finite verb comes early in the clause. But after a subordinating conjunction like selv om, that main-clause pattern disappears.

So this is natural Danish subordinate-clause word order:

• selv om emnet først virkede meget kompliceret

A helpful comparison:

• Main clause: Først virkede emnet meget kompliceret.
• Subordinate clause: ... selv om emnet først virkede meget kompliceret.

So the placement of først before virkede fits the subordinate clause structure.

Here først means at first or initially.

So it does not mean first in the sense of first, second, third.
Instead, it describes how something seemed in the beginning.

So:

• emnet først virkede meget kompliceret
  = the subject initially seemed very complicated

This is a very common use of først in Danish.

In this sentence, virkede means seemed or appeared.

The verb virke can have different meanings depending on context:

• Maskinen virker. = The machine works.
• Det virker mærkeligt. = It seems strange.

Here it is used with an adjective:

• virkede meget kompliceret = seemed very complicated

So in this sentence, it definitely means seemed, not worked.

Because meget is an adverb here, meaning very.

It modifies the adjective kompliceret:

• kompliceret = complicated
• meget kompliceret = very complicated

This is the normal Danish pattern:

• meget interessant = very interesting
• meget svært = very difficult
• meget godt = very good

So meget here does the same job as very in English.

Because the adjective comes after the verb virkede and describes the subject. That is called a predicative adjective.

Compare:

• et kompliceret emne = a complicated subject
• emnet virkede kompliceret = the subject seemed complicated

In the sentence you gave, the adjective is not placed before the noun; it comes after virkede, so kompliceret is the correct form.

komplicerede would be used in forms like:

• de komplicerede emner = the complicated subjects
• komplicerede problemer = complicated problems

So in your sentence, kompliceret is the normal singular form after virkede.