Hendes opførsel over for den nye kollega var venlig, og derfor blev stemningen med det samme bedre.

Because sin/sit/sine can only refer back to the subject of the clause.

In this sentence, the grammatical subject is opførsel (behavior), not hun (she). So Danish uses hendes.

• Hendes opførsel var venlig = Her behavior was kind
• Hun ændrede sin opførsel = She changed her own behavior

So a native English speaker often has to remember that Danish reflexive possessives depend on the grammar of the clause, not just on who logically “owns” something.

Opførsel means behavior or conduct.

It is a normal, common noun and is often used in contexts like:

• god opførsel = good behavior
• dårlig opførsel = bad behavior
• hans opførsel over for andre = his behavior toward others

In this sentence, it refers to how she acted toward the new colleague.

Over for is a common Danish prepositional expression meaning toward, towards, or in relation to someone.

Here:

• opførsel over for den nye kollega = behavior toward the new colleague

You will also see it with adjectives:

• venlig over for nogen = kind to/toward someone
• uhøflig over for nogen = rude to/toward someone

So over for is something worth learning as a fixed expression.

When Danish has a definite noun phrase with an adjective, it normally uses:

• den/det/de + adjective + noun

So:

• den nye kollega = the new colleague

Not:

• den nye kollegaen ❌

This is different from English and also different from some other Scandinavian languages. In Danish, after den/det/de + adjective, the noun is normally in its basic form, not the -en/-et definite form.

Compare:

• kollegaen = the colleague
• den nye kollega = the new colleague

Because adjectives usually take -e in a definite noun phrase.

So:

• en ny kollega = a new colleague
• den nye kollega = the new colleague

That -e form is very common after den, det, de.

No. Den shows grammatical gender, not biological sex.

In Danish, kollega is a common-gender noun, so it takes den:

• den kollega

But the person can be male, female, or any gender. Grammatical gender in Danish is mostly about how words behave grammatically, not about real-world gender.

Because venlig is a predicative adjective here, and it agrees with opførsel, which is a common-gender singular noun.

So:

• opførsel var venlig = common gender singular → venlig
• If the subject were neuter, you would often get -t:
  • svaret var venligt = the answer was kind/polite

A useful contrast:

• en venlig person = a kind person
• et venligt svar = a kind reply

Here the noun is opførsel, so venlig is correct.

Because Danish main clauses follow the V2 rule: the finite verb must come in the second position.

In the second clause:

• og = and (this does not count as part of the clause itself)
• derfor = first element in the clause
• blev = finite verb, so it must come next
• stemningen = subject

So the order is:

• derfor blev stemningen ...

not:

• derfor stemningen blev ... ❌

This is a very common Danish word-order pattern:

• Derfor kom han sent
• Så gik vi hjem
• I går så jeg hende

No. Blev is the past tense of blive, and here it means became or got.

So:

• stemningen blev bedre = the atmosphere became better / got better

That is different from:

• stemningen var bedre = the atmosphere was better

So blev shows a change of state, not just a description.

Because it refers to the specific atmosphere/mood in that situation, not to some atmosphere in general.

Danish often uses the definite form when the context makes it clear which thing is meant. Here, the sentence is talking about the mood in the room/work situation that both speaker and listener can identify.

So:

• stemningen = the atmosphere / the mood

Med det samme is an idiomatic expression meaning:

• immediately
• right away
• at once

So:

• blev stemningen med det samme bedre = the atmosphere immediately became better

Even though the literal words are with the same, you should learn med det samme as a fixed expression.

Bedre is the comparative form of god (good).

Just like English has:

• good → better → best

Danish has:

• god → bedre → bedst

So:

• stemningen blev bedre = the atmosphere got better

You do not say mere god here.

Yes, that is possible.

Danish allows some flexibility in adverb placement, and these versions are all natural, though the emphasis shifts a little:

• ... og derfor blev stemningen med det samme bedre
• ... og stemningen blev derfor med det samme bedre

Putting derfor first gives it a bit more prominence: and therefore...

So the original sentence is perfectly natural, but not the only possible word order.