Jeg lægger et rent lagen på sengen, før min søster kommer på besøg.

Because lægge and ligge are different verbs in Danish:

• lægge = to lay / put / place something somewhere
• ligge = to lie / be lying somewhere

In this sentence, the speaker is actively putting the sheet onto the bed, so Danish uses lægger:

• Jeg lægger et rent lagen på sengen. = I put a clean sheet on the bed.

If the sheet were already there, you would use ligger:

• Et rent lagen ligger på sengen. = A clean sheet is lying on the bed.

So a useful shortcut is:

• lægge usually takes an object
• ligge usually describes position or state

Because lagen is a neuter noun in Danish, so it takes:

• the article et
• the neuter adjective ending -t

That gives:

• et lagen
• et rent lagen

Compare:

• en stor stol = a big chair
• et stort bord = a big table

Here:

• lagen is neuter
• ren becomes rent before a singular neuter noun

So:

• en ren skjorte = a clean shirt
• et rent lagen = a clean sheet

Danish adjectives change form depending on the noun.

For ren:

• ren = singular common gender
• rent = singular neuter
• rene = plural or definite use in many contexts

Since lagen is singular and neuter, the correct form is rent:

• et rent lagen

Examples:

• en ren seng = a clean bed
• et rent lagen = a clean sheet
• rene lagener = clean sheets

So rent is just agreement with et lagen.

Because på and i express different locations.

• på sengen = on the bed
• i sengen = in bed / in the bed

In this sentence, the sheet is being placed on the surface of the bed, so på is the natural choice.

Compare:

• Jeg lægger bogen på sengen. = I put the book on the bed.
• Barnet ligger i sengen. = The child is in bed.

So for a sheet being placed onto the bed, på sengen is correct.

Sengen is the definite form of seng:

• en seng = a bed
• sengen = the bed

Danish often uses the definite form when the thing is already understood from context. In a sentence like this, it is natural to say the bed, not just a bed, because the speaker and listener likely know which bed is meant.

This is very common in Danish:

• på bordet = on the table
• i køkkenet = in the kitchen
• på sengen = on the bed

Because Danish, like English, often uses the present tense for future events, especially when the future event is seen as planned, expected, or connected to a time word.

Here:

• før min søster kommer på besøg literally uses present tense
• but it refers to a future event: the sister’s upcoming visit

This is completely normal in Danish.

Examples:

• Jeg ringer i morgen. = I’ll call tomorrow.
• Hun kommer senere. = She’ll come later.
• Vi spiser, når han kommer. = We’ll eat when he comes.

So kommer does not mean the sister is coming right now; it simply fits normal Danish future reference.

På besøg is a fixed expression meaning:

• visiting
• on a visit
• for a visit

So:

• komme på besøg = to come visit / to come over

It works as an idiomatic phrase, so you do not normally insert an article there.

Examples:

• Hun er på besøg hos sin mor. = She is visiting her mother.
• De kommer på besøg i weekenden. = They are coming to visit on the weekend.

So in your sentence:

• min søster kommer på besøg = my sister is coming to visit / is coming over

Because før introduces a subordinate clause, and subordinate clauses in Danish normally keep the order:

• subject + verb

So:

• før min søster kommer på besøg

This is different from main clauses, where Danish often has verb-second word order.

Compare:

Main clause:

• I morgen kommer min søster på besøg.

Subordinate clause:

• før min søster kommer på besøg

A useful rule:

• after conjunctions like før, når, fordi, at, hvis, you usually get subordinate-clause word order

Because Danish normally uses a comma before a subordinate clause.

Here, før min søster kommer på besøg is a subordinate clause introduced by før, so the comma is standard:

• Jeg lægger et rent lagen på sengen, før min søster kommer på besøg.

This is one of the noticeable differences from English, where commas before before are not always used in the same way.

So the comma here is mainly a punctuation rule of Danish sentence structure.

Because Danish usually expresses possession with a possessive word before the noun:

• min søster = my sister
• din bror = your brother
• hendes bil = her car

The possessive agrees with the gender/number of the noun:

• min søster because søster is a common-gender singular noun
• mit lagen because lagen is a neuter singular noun
• mine bøger for plural

So:

• min søster
• mit lagen
• mine søstre

Usually no, or at least not in normal neutral Danish.

Danish does not use vil in the same general way English uses will for future. Very often, simple present is enough:

• min søster kommer på besøg = my sister is coming to visit / will come to visit

Adding vil often changes the meaning. It can suggest:

• willingness
• intention
• insistence
• sometimes a special nuance rather than simple future

So for a straightforward future event, kommer is the natural choice.

Yes. Danish present tense often covers both simple present and present progressive meanings that English separates.

So:

• Jeg lægger et rent lagen på sengen

can mean, depending on context:

• I put a clean sheet on the bed
• I am putting a clean sheet on the bed

Danish does not normally need a special form like English am putting. The context tells you whether it is habitual, immediate, or near-future.

Yes, but learners should know that it is a specific household word meaning sheet.

A few related words:

• lagen = sheet
• dynebetræk = duvet cover
• pudebetræk = pillowcase
• sengetøj = bed linen / bedding

So if you are talking specifically about the sheet on a bed, lagen is exactly the right word.

Yes. A few alternatives are possible, depending on style and nuance:

• Jeg lægger et rent lagen på sengen, inden min søster kommer på besøg.
• Jeg skifter lagenet, før min søster kommer på besøg.

Notes:

• før and inden can both mean before, though før is very common here.
• Jeg skifter lagenet means I’m changing the sheet, which may sound even more natural in some situations.

But your original sentence is perfectly understandable and natural Danish.