Learn Hardcore Croatian: U obitelji često mirno razgovaramo o djetinjstvu. - In the family we often talk calmly about childhood.

The base noun is obitelj (family, feminine).
The locative singular of obitelj is obitelji.
So u obitelji literally means in (the) family or, more naturally, in our family.

If you said u obitelj (accusative), it would mean into the family (movement towards), not in the family (location).

Which case are obitelji and djetinjstvu, and why are they in that form?

Both obitelji and djetinjstvu are in the locative singular.

obitelj → obitelji (locative sg.)
djetinjstvo → djetinjstvu (locative sg.)

In this sentence:

u obitelji – u
- locative = in the family (location)
o djetinjstvu – o
- locative = about childhood (topic)

The prepositions:

u (in) takes the locative when it means in/at (no movement).
o (about) always takes the locative to mark the topic of speaking, thinking, reading, etc.

Does u obitelji mean in the family or in my/our family?

Literally it is in the family, because Croatian has no articles. In context, though, u obitelji very often means in our family, in my family, or within the family in general.

So:

U obitelji često mirno razgovaramo…
is usually understood as
In our family, we often talk calmly…,
not as In some family, we often talk….

Context normally tells you whose family is meant.

Why is there no we (mi) in razgovaramo? How do I know it means we talk?

Croatian usually drops subject pronouns because they are clear from the verb ending.

The verb is razgovarati (to talk, to have a conversation).
Present tense endings (shortened):

ja razgovaram – I talk
ti razgovaraš – you talk
on/ona razgovara – he/she talks
mi razgovaramo – we talk
vi razgovarate – you (pl/formal) talk
oni/one razgovaraju – they talk

The ending -amo clearly marks 1st person plural (we), so the pronoun mi is usually unnecessary.
You could say Mi u obitelji često mirno razgovaramo…, but it adds emphasis on we and feels a bit heavier.

What is the difference between razgovaramo, govorimo, and pričamo?

All three can relate to speaking, but they’re used differently:

razgovaramo – we talk / we have a conversation
Emphasizes a two‑way exchange, a conversation between people.
U obitelji često mirno razgovaramo o djetinjstvu.
= We sit and discuss childhood together.
govorimo – we speak / we talk (in general, state, or language)
More neutral, often about the fact of speaking or about languages:
Govorimo o djetinjstvu. – We are talking about childhood.
Govorimo engleski. – We speak English.
pričamo – we tell / we chat
Often about telling stories, anecdotes, or chatting informally:
Pričamo o djetinjstvu. – We chat / tell stories about childhood.

In your sentence, razgovaramo fits well because it suggests a calm, mutual conversation.

Is mirno an adjective or an adverb here? Why doesn’t it agree in gender or number?

In this sentence, mirno is an adverb, describing how you talk: calmly / peacefully.

As an adjective, the base word is miran (calm).
Adjective examples:
- miran čovjek – a calm man
- mirna obitelj – a calm family
- mirno dijete – a calm child
As an adverb, the form is mirno and it does not change for gender or number:
- razgovaramo mirno – we talk calmly
- djeca mirno sjede – the children sit quietly

So there’s no agreement needed; mirno is in its fixed adverb form.

Can I move često and mirno around? For example: Često u obitelji mirno razgovaramo o djetinjstvu or U obitelji razgovaramo često mirno o djetinjstvu?

Croatian word order is fairly flexible, so several positions are possible. Native‑like options include:

U obitelji često mirno razgovaramo o djetinjstvu. (original)
Često u obitelji mirno razgovaramo o djetinjstvu.
U obitelji često razgovaramo mirno o djetinjstvu.
U obitelji mirno i često razgovaramo o djetinjstvu.

Typically:

Adverbs of frequency (like često = often) tend to be closer to the beginning or right before the verb.
Adverbs of manner (like mirno = calmly) are often placed close to the verb as well, usually just before or just after it.

Your very heavy version U obitelji razgovaramo često mirno o djetinjstvu is grammatical but sounds clumsy and unnatural because too many adverbs split things apart. Native speakers usually keep them grouped more tightly around the verb.

Why are both često and mirno in their base forms? Should they change for case, gender, or number?

No, they should not change. Both često (often) and mirno (calmly) are adverbs, and in Croatian adverbs do not inflect for case, gender, or number.

Examples:

Ona često putuje. – She often travels.
Oni često kasne. – They are often late.
On govori mirno. – He speaks calmly.
Djeca mirno spavaju. – The children sleep peacefully.

The form često stays često, and mirno stays mirno, regardless of who is doing the action.

Why is it o djetinjstvu and not something like o djetinjstvo or o djetinjstva?

Because the preposition o (about) always takes the locative case in standard Croatian.

The noun djetinjstvo (childhood) has:

nominative: djetinjstvo
genitive: djetinjstva
locative: djetinjstvu

Since o wants the locative, you must use djetinjstvu:

o djetinjstvu – about childhood

Forms like o djetinjstvo or o djetinjstva are incorrect in standard language.

What aspect is razgovaramo? Is there a perfective version?

razgovaramo is imperfective aspect.

Imperfective verbs describe ongoing, repeated, or open‑ended actions:
U obitelji često mirno razgovaramo o djetinjstvu.
= This is something we do regularly.

A common perfective partner is porazgovarati:

porazgovaramo – we will have a talk (once, with a sense of completion)
Večeras ćemo mirno porazgovarati o tome.
– This evening we’ll (have a proper) talk about it.

In your sentence, because it’s about a habitual action (often), the imperfective razgovaramo is the correct choice.

Could I say na obitelji instead of u obitelji?

No, not here. u and na are not interchangeable:

u = in, inside, within (physical or abstract space)
- u kući – in the house
- u obitelji – in the family / within the family
na = on, on top of, at (some surfaces, events, institutions, etc.)
- na stolu – on the table
- na poslu – at work
- na koncertu – at a concert

For the idea within the family context, Croatian uses u obitelji, not na obitelji.