Min søster lægger sin sovepose og sit underlag ind i teltet.

In Danish, you normally do not use the definite ending on a family noun when it already has a possessive word like min.

So:

• min søster = my sister
• søsteren = the sister

You usually choose one or the other:

• min søster
• søsteren

not min søsteren.

Here, lægger means puts, places, or lays down.

The verb is at lægge, which is used when someone places something somewhere.

So in this sentence, lægger is about actively moving the sleeping bag and sleeping pad into the tent.

A useful comparison is:

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

So:

• Hun lægger soveposen i teltet = she puts the sleeping bag in the tent
• Soveposen ligger i teltet = the sleeping bag is lying in the tent

Danish uses sin/sit/sine as a reflexive possessive when the owner is the same as the subject of the sentence.

Here, the subject is min søster, so if the sleeping bag and sleeping pad belong to the sister, Danish normally uses sin/sit:

• Min søster lægger sin sovepose ... = my sister puts her own sleeping bag ...
• Min søster lægger sit underlag ... = my sister puts her own sleeping pad ...

If you used hendes, it would usually suggest that the items belong to some other woman, not to the sister herself.

So the contrast is:

• sin/sit/sine = her own
• hendes = hers, but usually someone else’s in this kind of sentence

Because sin/sit/sine must agree with the noun they belong to.

• sovepose is a common gender noun, so you use sin
• underlag is a neuter noun, so you use sit

The basic pattern is:

• sin for en-words
• sit for et-words
• sine for plural nouns

So:

• sin sovepose
• sit underlag
• sine ting

Here underlag means something like a sleeping pad, ground pad, or mat used for camping.

Grammatically, it is a neuter noun in this sentence, which is why you see:

• et underlag
• sit underlag
• underlaget = the sleeping pad / the mat

Because teltet is the definite form: the tent.

In Danish, definiteness is often shown by adding an ending to the noun:

• et telt = a tent
• teltet = the tent

Since the sentence refers to a specific tent, Danish uses teltet.

Ind i means into.

It shows movement from outside to inside:

• ind i teltet = into the tent

This is different from just being located inside something. Compare:

• Hun går ind i teltet = she goes into the tent
• Hun er i teltet = she is in the tent

So in your sentence, ind i emphasizes that the sister is moving the items into the tent.

Yes. This is standard Danish main-clause word order:

• subject + verb + rest

So:

• Min søster = subject
• lægger = verb
• sin sovepose og sit underlag ind i teltet = the rest of the sentence

Danish is a V2 language, which means the finite verb usually comes in the second position in a main clause. In this sentence, the subject comes first, so the verb comes right after it.

It is repeated because the two nouns have different grammatical genders.

• sovepose needs sin
• underlag needs sit

So Danish cannot normally use just one possessive for both here.

That is why you get:

• sin sovepose og sit underlag

not simply one shared form before both nouns.

Yes. Lægger is present tense.

Like English present tense in some contexts, Danish present tense can often be translated in more than one way depending on context:

• My sister puts her sleeping bag and her sleeping pad into the tent
• My sister is putting her sleeping bag and her sleeping pad into the tent

Without extra context, both can be possible translations.

Not with the same meaning.

• ind i teltet = into the tent, focusing on movement
• inde i teltet = inside the tent, focusing on location

Since the sentence is about moving the items from one place to another, ind i teltet is the natural choice.

If the sentence were about where the items already are, then inde i could make sense.

Og simply means and.

It connects the two objects:

• sin sovepose
• sit underlag

So the sister is putting both items into the tent.