Jeg sætter min underskrift nederst på papiret, før jeg afleverer det.

Sætter is the present tense form of the verb at sætte.

• at sætte = to put / to place / to set
• jeg sætter = I put / I am putting

In Danish, the infinitive often ends in -e, while the present tense usually ends in -r:

• at skrive → jeg skriver
• at aflevere → jeg afleverer
• at sætte → jeg sætter

So Jeg sætter ... is the correct finite verb form for I put / I place ....

Danish often says sætte sin underskrift for to put one’s signature.

This is a very natural expression, similar to English put your signature or sign.
You may also hear related phrases such as:

• skrive under = sign
• sætte sin underskrift på noget = put one’s signature on something

So Jeg sætter min underskrift ... is idiomatic Danish, not just a word-for-word choice.

Because in Danish, a possessive like min, din, hans, vores, etc. normally replaces the definite ending.

So:

• en underskrift = a signature
• underskriften = the signature
• min underskrift = my signature

You do not usually combine min with the definite ending here.
That is why min underskrift is correct, while min underskriften is not.

Underskrift means signature.

It is a common noun in formal and everyday Danish, especially in contexts involving:

• documents
• forms
• contracts
• official papers

Related expressions:

• at skrive under = to sign
• min underskrift = my signature
• din underskrift mangler = your signature is missing

Nederst means at the bottom or lowest down.

In this sentence, nederst på papiret means at the bottom of the paper.

It works as an adverb describing location. Compare:

• øverst = at the top
• nederst = at the bottom

Examples:

• Skriv dit navn øverst. = Write your name at the top.
• Sæt din underskrift nederst. = Put your signature at the bottom.

Because Danish uses på when something is written on the surface of paper.

• på papiret = on the paper
• i papiret would suggest something inside the paper, which does not fit here

So if you write, print, or sign something, Danish normally uses på:

• skrive på papiret = write on the paper
• tegne på papiret = draw on the paper
• sætte sin underskrift på papiret = put one’s signature on the paper

The ending -et is the definite article attached to the noun.

• et papir = a paper
• papiret = the paper

Danish often puts the at the end of the noun instead of using a separate word like English does.

Since papir is a neuter noun (et noun), its definite singular form is -et:

• et hus → huset
• et brev → brevet
• et papir → papiret

Det refers back to papiret or, more broadly, the document/paper being handed in.

Since papir is a neuter noun (et papir), the matching pronoun is det:

• papiret → det

So:

• jeg afleverer det = I hand it in / I submit it

If the noun were a common-gender (en) noun, the pronoun would often be den instead.

Because før jeg afleverer det is a subordinate clause, and subordinate clauses do not follow the normal main-clause verb-second pattern.

Main clause:

• Jeg afleverer det.

Subordinate clause after før:

• før jeg afleverer det

So after conjunctions like før, fordi, at, når, hvis, Danish normally keeps the order:

conjunction + subject + verb + ...

Examples:

• før jeg går
• fordi han arbejder
• når vi kommer

This is why før afleverer jeg det would be wrong here.

Yes. Both verbs are in the present tense:

• sætter
• afleverer

Danish often uses the present tense for:

• habitual actions
• general procedures
• near-future actions
• instructions or descriptions of what someone does next

So the sentence can mean something like:

• I put my signature at the bottom of the paper before handing it in
• I will put my signature at the bottom of the paper before handing it in

The exact time often comes from context rather than from a special future tense form.

Yes, in many contexts inden would also work.

• før = before
• inden = before

In a sentence like this, both can sound natural:

• før jeg afleverer det
• inden jeg afleverer det

That said, usage can vary a little by region, style, and context. In everyday Danish, før is very common and completely natural here.

Because Danish comma rules commonly place a comma before a subordinate clause.

Here, før jeg afleverer det is a subordinate clause introduced by før, so the comma marks that boundary:

• Jeg sætter min underskrift nederst på papiret, før jeg afleverer det.

You will often see this with words such as:

• fordi
• at
• når
• hvis
• før

Comma use can vary a bit depending on the comma system being followed, but this comma is standard and very normal.

Yes. At aflevere is a very common verb meaning to hand in, to submit, to deliver, or to turn in, depending on context.

Examples:

• aflevere en opgave = hand in an assignment
• aflevere en rapport = submit a report
• aflevere nøglerne = hand over the keys

So in this sentence, jeg afleverer det naturally means I hand it in or I submit it.