U studenome i prosincu dani su kraći, pa navečer češće ostajemo u toploj sobi.

They are in the locative case.

After u meaning in, Croatian often uses the locative for:

• place: u sobi = in the room
• time period: u prosincu = in December

So:

• studeni → u studenome / u studenom
• prosinac → u prosincu

A useful thing to notice is that the two month names decline differently:

• studeni behaves like an adjective-based word
• prosinac behaves like a regular masculine noun

So the different endings are normal.

Yes. Both u studenome and u studenom are correct.

The longer ending -ome is a full form, while -om is a shorter form. In everyday speech, many speakers use u studenom, but u studenome is also standard and perfectly natural.

Because the sentence is making a general statement about days in those months, not talking about one specific day.

So Croatian uses the plural:

• dani = days
• su = are
• kraći = shorter

The adjective must agree with dani, so it is also masculine plural:

• singular: dan je kraći
• plural: dani su kraći

Kraći is the comparative of kratak (short).

So:

• kratak = short
• kraći = shorter

Here it agrees with dani, so it is in the masculine plural form.

In English, we might say the days are shorter. Croatian does the same idea with the comparative adjective kraći.

Here pa means something like:

• so
• and so
• therefore
• which is why

It connects the first idea to the consequence:

• U studenome i prosincu dani su kraći
  In November and December the days are shorter
• pa navečer češće ostajemo u toploj sobi
  so in the evening we stay in the warm room more often

It is a very common, natural connector in Croatian.

Navečer is an adverb, and it means in the evening or in the evenings.

It is written as one word:

• navečer = in the evening

In this sentence it has a general, habitual meaning:

• navečer češće ostajemo... = we more often stay... in the evenings

So it is not just one particular evening, but a usual pattern.

Because češće is an adverb, and it modifies the verb ostajemo.

It tells us how often we stay:

• ostajemo češće = we stay more often

Compare:

• češći = adjective, more frequent
• češće = adverb, more often

So:

• češći posjeti = more frequent visits
• češće ostajemo = we stay more often

This is mainly about aspect.

• ostajemo comes from the imperfective verb ostajati
• ostanemo comes from the perfective verb ostati

In this sentence, the meaning is habitual/repeated:

• in November and December, we tend to stay inside more often

For repeated or ongoing situations, Croatian normally uses the imperfective, so ostajemo is the natural choice.

If you used ostanemo, it would sound more like a single completed event, which does not fit this general seasonal statement.

Croatian often drops subject pronouns when they are already clear from the verb ending.

Here:

• ostajemo = we stay

The ending -mo already shows 1st person plural, so mi is not necessary.

You could say:

• mi ostajemo

but it would usually sound more emphatic, like we stay.

Because after u meaning in for location, Croatian uses the locative case.

So both the adjective and the noun must be in locative singular feminine:

• topla soba = a warm room
• u toploj sobi = in a warm room

Breakdown:

• topla → toploj
• soba → sobi

This is normal agreement: the adjective changes to match the noun in case, gender, and number.

This is the classic location vs. motion difference with u:

• u toploj sobi = in the warm room
  
  • location
  • uses locative
• u toplu sobu = into the warm room
  
  • movement toward/into
  • uses accusative

So in your sentence, the meaning is staying inside the room, not going into the room, which is why u toploj sobi is used.

Croatian word order is fairly flexible, but the version in the sentence is very natural.

Current order:

• U studenome i prosincu dani su kraći, pa navečer češće ostajemo u toploj sobi.

You could also hear variations such as:

• U studenome i prosincu dani su kraći, pa češće navečer ostajemo u toploj sobi.
• Navečer češće ostajemo u toploj sobi...

But the original order sounds smooth and natural because it presents the ideas clearly:

1. time period
2. statement about the days
3. consequence
4. usual time of day
5. action
6. place