Læreren nævner også, at datoen mangler øverst på siden.

Because læreren is the definite singular form of en lærer.

• en lærer = a teacher
• læreren = the teacher

In Danish, the definite article is usually attached to the end of the noun instead of being a separate word like English the.

Nævner is the present tense of at nævne.

• at nævne = to mention
• nævner = mentions / is mentioning

Danish present tense is often made by adding -r to the infinitive, so nævne → nævner.

Også means also or too.

In this sentence it comes after the finite verb nævner, which is a very normal position in a Danish main clause:

• Læreren = subject
• nævner = finite verb
• også = adverb

That placement fits the usual Danish verb-second pattern in main clauses.

The comma marks the beginning of a subordinate clause: at datoen mangler øverst på siden.

In Danish, many writers use a comma before subordinate clauses like this. You may also see sentences without that comma, because modern Danish allows both systems:

• Læreren nævner også, at datoen mangler øverst på siden.
• Læreren nævner også at datoen mangler øverst på siden.

So the comma is normal, but its use can depend on the comma style being followed.

Here at means that and introduces a subordinate clause:

• ... nævner også, at ... = ... also mentions that ...

This is not the infinitive-marker at used before verbs, as in:

• at skrive = to write

So Danish at can have different jobs. In this sentence, it is a conjunction meaning that.

Datoen means the date, not a date.

• en dato = a date
• datoen = the date

The definite form is used because it refers to a specific date that is expected to be there, for example on the top of a page. It is not just any date in general.

Mangler comes from at mangle, which means to be missing or to be lacking.

In Danish, this verb is often used where English would say is missing:

• Datoen mangler = The date is missing

So even though English often uses be + missing, Danish commonly uses the active verb mangle.

Øverst på siden means at the top of the page.

Breakdown:

• øverst = at the top / uppermost
• på = on
• siden = the page / the side

Here side means page, which is very common in Danish. So the phrase is idiomatic Danish for saying something is located at the top of a page.

Yes:

Læreren | nævner | også, | at | datoen | mangler | øverst på siden

Main clause:

• Læreren = subject
• nævner = finite verb
• også = adverb

Subordinate clause introduced by at:

• datoen = subject
• mangler = verb
• øverst på siden = place expression

So the basic structure is:

The teacher + mentions + also + that + the date + is missing + at the top of the page

Yes. Danish side can mean both side and page, depending on context.

For example:

• på den anden side = on the other side
• på side 5 = on page 5

In this sentence, because we are talking about where a date should appear on a written sheet or page, siden is understood as the page.