Ég hefði átt að athuga skjátextann og hljóðnemann áður en ég byrjaði myndina.

In Icelandic, the is usually attached to the end of the noun instead of written as a separate word.

So here:

• skjátextann = the subtitle / the caption track
• hljóðnemann = the microphone
• myndina = the movie

This attached article changes form depending on gender, number, and case.

This is a very common Icelandic structure:

hefði átt að + infinitive

It expresses something that would have been the right thing to do in the past, often with the idea that it probably did not happen.

So:

• Ég hefði átt að athuga = I should have checked

A more literal sense is something like I would have ought to check, but in natural English the best translation is I should have checked.

Átt is the past participle of eiga.

The verb eiga normally means to own, but in the construction eiga að + infinitive, it means something like:

• to be supposed to
• to ought to
• to have to

So in this sentence, átt is not about possession. It is part of the obligation expression hefði átt að.

Because after eiga að and hefði átt að, Icelandic uses að + infinitive.

Here:

• athuga = check
• að athuga = to check

So:

• hefði átt að athuga = should have checked

This is just the normal pattern of the construction.

Athuga is the infinitive of the verb, meaning to check.

It stays in the infinitive because it comes after hefði átt að:

• Ég hefði átt að athuga...
• literally: I should have to check...
• naturally: I should have checked...

So even though English uses checked, Icelandic keeps athuga in the infinitive.

They are both:

• masculine
• singular
• definite
• accusative

They are in the accusative because they are the direct objects of athuga.

Dictionary forms:

• skjátexti = subtitle, caption
• hljóðnemi = microphone

In this sentence, they become:

• skjátextann
• hljóðnemann

So the ending is telling you both the and the case.

Because myndina is the accusative singular definite form, and it is the direct object of byrjaði.

• mynd = movie, film, picture
• myndin = the movie as a subject
• myndina = the movie as an object

Here the speaker started the movie, so the noun is an object:

• ég byrjaði myndina

That is why Icelandic uses myndina, not myndin.

Yes, skjátextann is grammatically singular.

Depending on context, English might translate it as the subtitle, the caption track, or more naturally the subtitles. Languages do not always match perfectly in number.

So the important thing for the learner is:

• Icelandic form here: singular
• English translation may sometimes sound more natural in the plural

Áður en means before.

It introduces a clause:

• áður en ég byrjaði myndina = before I started the movie

It is best to learn áður en as a fixed expression.

It is the same written word, but here it is part of the expression áður en.

So in this sentence, do not think of en by itself as but. Learn the whole phrase:

• áður en = before

That is the most useful way to understand it.

Because áður en ég byrjaði myndina is a full subordinate clause with its own subject and verb.

So Icelandic, like English, says:

• before I started the movie

not just

• before started the movie

The second ég is needed because byrjaði has its own subject.

Because the clause after áður en refers to an event that actually happened in the past:

• I started the movie

Meanwhile, the first part expresses hindsight:

• I should have checked...

So the contrast is:

• unrealized or regretted obligation: hefði átt að athuga
• actual past event: byrjaði myndina

That is why the simple past byrjaði is natural here.

Yes, but the meaning changes.

• Ég átti að athuga... = I was supposed to check...
• Ég hefði átt að athuga... = I should have checked...

The version with hefði sounds more like hindsight, regret, or criticism after the fact. It often suggests that the speaker now realizes the check should have been done.

The sentence follows a very normal Icelandic pattern:

• Ég = subject
• hefði átt að athuga = verb phrase
• skjátextann og hljóðnemann = objects
• áður en ég byrjaði myndina = time clause

So the structure is:

Subject + verb phrase + objects + subordinate clause

Nothing especially unusual is happening with the word order here; it is quite natural and neutral.

Yes. They are two coordinated direct objects joined by og:

• athuga skjátextann
• athuga hljóðnemann

Together:

• athuga skjátextann og hljóðnemann

So the speaker should have checked both things before starting the movie.