U našem starom stanu bio je običaj da večeramo u kuhinji, a ovdje ćemo češće jesti u dnevnom boravku.

Because u here means in, indicating location, so it takes the locative case.

Breakdown:

• u = in
• našem = our, locative singular masculine/neuter
• starom = old, locative singular masculine/neuter
• stanu = apartment/flat, locative singular of stan

So:

• stan = apartment
• u stanu = in the apartment
• u našem starom stanu = in our old apartment

Both našem and starom must agree with stanu in case, gender, and number.

Bio je običaj da... means something like:

• it was customary to...
• it was the habit/practice to...
• it used to be the custom that...

Literally:

• bio je = was
• običaj = custom, habit, practice

So bio je običaj da večeramo u kuhinji is literally it was the custom/habit that we eat dinner in the kitchen, but in natural English you would usually say it was customary for us to have dinner in the kitchen or we used to have dinner in the kitchen.

In Croatian, da + present tense is very commonly used after expressions like it is usual, it is important, it is possible, it was a habit/custom, and so on.

So:

• bio je običaj da večeramo = it was customary that we have dinner / it was customary for us to have dinner

Here da introduces a subordinate clause. English often uses an infinitive here, but Croatian often prefers da + present.

This does not mean a present-time action. It is simply the normal structure after običaj in this kind of sentence.

Because after da, Croatian usually uses a finite verb form rather than the infinitive.

So:

• da večeramo = that we have dinner
• not da večerati

Večeramo is the 1st person plural present tense of večerati:

• ja večeram = I have dinner
• mi večeramo = we have dinner

Even though the whole sentence refers partly to the past, the verb after da still appears in the present form because that is how this construction works.

Both relate to eating, but they are not exactly the same.

• večerati / večeramo = to have dinner / we have dinner
• jesti = to eat

So the first part is more specific:

• da večeramo u kuhinji = that we have dinner in the kitchen

The second part is a bit more general:

• češće jesti u dnevnom boravku = eat more often in the living room

In practice, jesti here can still refer to meals generally, possibly including dinner, but večerati specifically refers to the evening meal.

That is the normal Croatian future tense.

It is formed with:

• a form of htjeti used as an auxiliary: ću, ćeš, će, ćemo, ćete, će
• plus the infinitive

So:

• ćemo jesti = we will eat

In this sentence:

• ovdje ćemo češće jesti u dnevnom boravku = here we will eat more often in the living room

The auxiliary ćemo is in second position, which is normal Croatian word order for clitics.

Češće means more often.

It is the comparative form of the adverb često = often.

So:

• često = often
• češće = more often

In the sentence:

• ovdje ćemo češće jesti = here we will eat more often

It compares the new situation with the old one: in the old apartment dinner was in the kitchen, but here eating in the living room will happen more often.

Again, because u means in and expresses location, so it takes the locative case.

Breakdown:

• dnevni boravak = living room
• u dnevnom boravku = in the living room

Form by form:

• dnevni → dnevnom
• boravak → boravku

Both words change because the adjective must agree with the noun in the locative singular masculine.

Ovdje means here.

It contrasts the present location with the previous one:

• u našem starom stanu = in our old apartment
• ovdje = here, in this place

So the sentence is contrasting:

• what was customary there before
• what will happen here now

Croatian often uses this kind of adverb to make the contrast very clear.

Because Croatian often leaves out subject pronouns when the verb already shows who the subject is.

Here:

• večeramo already means we have dinner
• ćemo jesti already means we will eat

So mi = we is unnecessary unless you want emphasis.

You could say mi ćemo češće jesti, but that would add emphasis, something like we, for our part, will eat more often...

Because ćemo is a clitic-like auxiliary and normally comes in the second position of its clause.

So:

• ovdje ćemo češće jesti = natural
• ovdje češće ćemo jesti = not the usual standard order

Croatian often places short unstressed words such as sam, si, je, smo, ste, su, ću, ćeš, će, ćemo... near the beginning of the clause, typically after the first stressed element.

Here the first element is ovdje, so ćemo follows it:

• ovdje ćemo...

Yes. Croatian has several natural ways to express this idea.

For example:

• U našem starom stanu obično smo večerali u kuhinji.
  = In our old apartment, we usually had dinner in the kitchen.
• U našem starom stanu bio je običaj večerati u kuhinji.
  = In our old apartment, it was customary to have dinner in the kitchen.
• U našem starom stanu bio je običaj da večeramo u kuhinji.
  = In our old apartment, it was customary for us to have dinner in the kitchen.

The version with bio je običaj da... sounds a bit more formal or descriptive than simply saying obično smo večerali.

No.

• stan = apartment / flat
• kuća = house

So u našem starom stanu specifically means in our old apartment/flat, not in our old house.

That distinction is important in Croatian, just as it is in English.

Yes. A here means something like and, while, or whereas, depending on context.

It links two contrasting situations:

• before: u našem starom stanu ... u kuhinji
• now/here: ovdje ... u dnevnom boravku

So a is not just simple addition. It often signals a mild contrast:

• ..., a ovdje ćemo... = ..., whereas here we will...

That makes the sentence feel like a comparison between old habits and the new living arrangement.