Det var først da jeg læste teksten højt, at jeg opdagede, at et navneord stod i flertal, selv om resten af sætningen var i ental.

This is an emphatic/cleft-like structure in Danish. It highlights the time when the discovery happened.

• Det var først da jeg læste teksten højt, at jeg opdagede ... = It was only when I read the text aloud that I noticed ...

The important part is først da = only when / not until.

A more neutral version would be something like:

• Jeg opdagede det først, da jeg læste teksten højt.

So the original sentence is not just stating the fact; it is focusing attention on the moment of discovery.

Here da refers to a specific event in the past:

• da jeg læste teksten højt = when I read the text aloud

Danish usually uses:

• da for one completed event in the past
• når for repeated events, general truths, or future situations

So here da is correct because the speaker is talking about one particular moment.

Compare:

• Da jeg kom hjem, ringede hun.
  = one past occasion

• Når jeg kommer hjem, ringer hun ofte.
  = habitual or general

That at is part of the structure Det var ... at ....

In this pattern, Danish often uses:

• Det var X, at Y
• Det er X, at Y

So:

• Det var først da jeg læste teksten højt, at jeg opdagede ...

The at introduces the clause containing the main point: jeg opdagede ...

It is not the same as English infinitive to. Here it is a subordinating conjunction meaning that.

Because opdage can be followed by a content clause:

• jeg opdagede, at ... = I discovered/noticed that ...

So the sentence contains two different clause introductions:

1. ..., at jeg opdagede ...
   part of the Det var ... at ... structure

2. jeg opdagede, at et navneord stod i flertal ...
   the thing that was discovered

This may feel heavy in English, but it is completely normal in Danish.

Højt is an adverb here, meaning aloud / out loud.

The common expression is:

• at læse højt = to read aloud

So:

• jeg læste teksten højt = I read the text aloud

Even though høj means high/loud, the form højt is used adverbially here.

Very common patterns:

• læse højt = read aloud
• sige noget højt = say something out loud

Teksten is the definite form of tekst:

• en tekst = a text
• teksten = the text

The speaker is talking about a specific text, presumably one already known from the context. That is why Danish uses the definite form.

This is very normal in Danish: instead of a separate word for the, Danish often adds the definite ending to the noun:

• bog → bogen
• tekst → teksten
• sætning → sætningen

Yes. Navneord is a neuter noun, so it takes et in the singular indefinite:

• et navneord

Danish nouns are divided into two grammatical genders:

• common gender: en
• neuter: et

So learners simply have to learn the gender with the noun.

Also, note that navneord is a school-style grammar term. You may also see:

• substantiv

Both refer to a noun, but navneord is often more learner-friendly or traditional in school contexts.

This is a very natural Danish way to talk about how a word appears in a text.

• stå i flertal = be in the plural / appear in the plural
• stå i ental = be in the singular / appear in the singular

The verb stå literally means stand, but Danish often uses it more abstractly for written forms or wording:

• Der stod et navn på listen. = a name was written on the list
• Ordet stod i flertal. = the word was in the plural

So this is idiomatic Danish, not something you should translate word-for-word into English.

Resten af ... is a very common Danish structure meaning the rest of ...

So:

• resten af sætningen = the rest of the sentence

This is more idiomatic than trying to build a possessive-style phrase.

Other examples:

• resten af dagen = the rest of the day
• resten af bogen = the rest of the book
• resten af klassen = the rest of the class

So this is a useful chunk to learn as a set phrase: resten af + noun.

Because i flertal and i ental are fixed grammatical expressions.

• i flertal = in the plural
• i ental = in the singular

Danish often leaves out an article in this kind of set phrase, especially when talking about:

• grammar
• categories
• states
• abstract labels

So you should learn these as complete expressions rather than trying to build them word by word.

Because these are subordinate clauses, and subordinate clauses in Danish normally do not use the main-clause verb-second pattern.

That means after words like:

• da
• at
• selv om

you normally get straightforward subject-verb order:

• da jeg læste
• at jeg opdagede
• selv om resten af sætningen var i ental

This is different from a main clause, where Danish usually puts the finite verb in second position:

• Jeg læste teksten højt.
• Så opdagede jeg det.

So this sentence is a good example of the contrast between main-clause word order and subordinate-clause word order.

Here selv om means although / even though.

It introduces a subordinate clause that expresses contrast:

• selv om resten af sætningen var i ental = although the rest of the sentence was in the singular

So the contrast is:

• one noun was plural
• but the rest of the sentence was singular

That is exactly the kind of situation where Danish uses selv om.

Because the sentence contains several subordinate clauses, and Danish punctuation often marks them clearly.

The commas separate these parts:

• Det var først da jeg læste teksten højt,
• at jeg opdagede,
• at et navneord stod i flertal,
• selv om resten af sætningen var i ental.

In other words, the commas are helping you see the clause structure.

A learner should especially notice that the sentence has:

• a da-clause
• an at-clause
• another at-clause
• a selv om-clause

So the commas are not random; they reflect the grammar of the sentence.

Because the whole sentence is describing a past event.

The speaker is looking back and telling you about the moment when they noticed the mistake. So the verbs match that past-time frame:

• var = was
• læste = read
• opdagede = discovered/noticed
• stod = was/appeared

Even the description of the noun being plural and the rest being singular is framed as part of what the speaker noticed at that time, so Danish keeps the narrative in the past.