Det er kun en uge siden, min tante fik gips på benet, så hun låner nu min søster en krykke.

This is a very common Danish pattern for talking about how much time has passed since something happened.

• Det er kun en uge siden ... = It is only a week since ...
• Literally, Danish uses it is + time expression + since

So:

• Det er en uge siden = It’s been a week since
• Det er kun en uge siden = It’s only been a week since

The verb in the main part is present tense er, even though the event after siden happened in the past.

Because min tante fik gips på benet is a subordinate clause introduced by siden.

So the structure is:

• Det er kun en uge siden, ...
• ... min tante fik gips på benet

In modern Danish, this comma is often optional depending on the comma style being used. Many writers include it; many also write:

• Det er kun en uge siden min tante fik gips på benet ...

So if you see the comma here, it is not strange.

Fik is the past tense of få, which usually means get or receive.

So:

• få = get
• fik = got

In this sentence:

• min tante fik gips på benet = my aunt got a cast on her leg

This is a very natural way in Danish to describe someone ending up with a cast.

Because gips is usually treated as an uncountable material noun, like plaster in English.

So Danish says:

• få gips på benet = get a cast on your leg

Even though English often uses a cast, Danish commonly uses just gips without an article in this kind of phrase.

You may also see more specific words in other contexts, but gips på benet is very normal everyday Danish.

Danish often uses the definite form with body parts when it is already obvious whose body part is meant.

So instead of saying:

• on her leg

Danish very often says:

• på benet = on the leg

This is especially common after prepositions and in everyday expressions about the body.

Compare:

• Han vasker hænderne = He washes his hands
• literally: He washes the hands

So fik gips på benet is completely natural Danish.

Both are possible, but they are not exactly the same in emphasis.

• en uge = a week / one week in a neutral way
• én uge = one week with extra emphasis on the number

In this sentence, kun en uge is the normal, neutral wording: only a week.

If you wrote kun én uge, it would stress that it was specifically one week and not more.

Here så means so or therefore.

It connects the two ideas:

• It has only been a week since her leg was put in a cast,
• so she is now lending my sister a crutch.

So this is not the time word så meaning then. It is the result/logic word:

• så = so / therefore

Because så here is a coordinating conjunction, so the clause after it is still a normal main clause.

That means normal main-clause word order applies:

• hun = subject
• låner = finite verb
• nu = adverb

So:

• så hun låner nu min søster en krykke

is built like an ordinary statement.

A useful contrast:

• så hun låner nu ... = so she is now lending ...
• derfor låner hun nu ... = therefore she is now lending ...

With derfor, the word order changes because derfor takes the first position in the clause. With så, it does not work that way.

In a normal Danish main clause, when the subject comes first, the finite verb usually comes second.

So the basic pattern is:

• hun låner nu ...

not:

• hun nu låner ...

That is why nu comes after låner here.

Very roughly:

• hun = 1st element
• låner = 2nd element
• nu = later in the clause

This is part of the usual Danish V2 word order in main clauses.

Here it means lends.

This is a famous Danish point: låne can mean both borrow and lend, depending on the structure.

Compare:

• Jeg låner en bog af ham = I borrow a book from him
• Jeg låner ham en bog = I lend him a book

In your sentence:

• hun låner nu min søster en krykke

there are two objects:

• min søster = the person receiving it
• en krykke = the thing being lent

So the meaning must be she is now lending my sister a crutch.

Because låne can take an indirect object directly, just like English lend someone something.

So Danish can say:

• låne nogen noget
• literally: lend someone something

That is why:

• hun låner nu min søster en krykke

is perfectly natural.

You could think of the structure as:

• hun = subject
• låner = verb
• min søster = indirect object
• en krykke = direct object

Because the sentence is talking about a current situation.

The first event happened in the past:

• min tante fik gips på benet

The second action is happening now:

• hun låner nu min søster en krykke

So Danish uses:

• past tense for the earlier event
• present tense for the current action

That combination is completely natural.