Learn Hardcore Croatian: Brat i sestra su se jučer posvađali, ali su se danas pomirili. - The brother and sister had an argument yesterday, but they made up today.

auxiliary verb biti (to be) in the present tense
- su = 3rd person plural (oni/one/ona)
active past participle (glagolski pridjev radni)
of the main verb
- posvađali – from posvađati (se)
- pomirili – from pomiriti (se)

So:

oni su se posvađali – they quarreled / they had a fight
oni su se pomirili – they made up / reconciled

English usually has just one word (argued, made up), but Croatian past tense is built from an auxiliary (su) + participle (posvađali, pomirili) + in this case the reflexive se.

Why is it posvađali and pomirili (masculine plural) when one of the subjects is female (sestra)?

In Croatian, when you have a mixed group (male + female), the masculine plural form is used by default.

brat (brother) – masculine
sestra (sister) – feminine
→ together: brat i sestra = grammatically masculine plural

So the participles must agree in number and gender with this compound subject:

Brat i sestra su se jučer posvađali.
Brat i sestra su se danas pomirili.

Compare:

Sestre su se jučer posvađale, ali su se danas pomirile.
(all female → feminine plural: posvađale, pomirile)

What does se do in su se jučer posvađali and su se danas pomirili? Can we leave it out?

Se is a reflexive clitic pronoun. Here, it is part of reflexive verbs:

posvađati se (s nekim) – to quarrel, to have a fight (with someone)
pomiriti se (s nekim) – to make up, to reconcile (with someone)

In this sentence, se is not optional; it is part of the normal dictionary form of the verbs. Without se, the meaning changes or the sentence becomes incomplete:

pomiriti nekoga (s nekim) – to reconcile somebody (with somebody)
- Roditelji su pomirili djecu. – The parents reconciled the children.

But here, the brother and sister reconcile themselves / each other, so Croatian uses the reflexive form:

Brat i sestra su se pomirili.

So se must be there with both verbs to express “they quarreled (with each other)” and “they made up (with each other)”.

Why is se repeated with both verbs: su se jučer posvađali, ali su se danas pomirili? Could we say Brat i sestra su se jučer posvađali, ali su danas pomirili?

Each reflexive verb needs its own se in its own clause.

First clause: (Brat i sestra) su se jučer posvađali.
Second clause: (Brat i sestra) su se danas pomirili.

If you say:

… ali su danas pomirili

this expects an object:

… ali su danas pomirili prijatelje. – they reconciled their friends

Without se, pomirili means “reconciled (somebody/something)”, not “made up (with each other)”.

So you must repeat se:

Brat i sestra su se jučer posvađali, ali su se danas pomirili.

You may drop the repeated subject in the second clause:

Brat i sestra su se jučer posvađali, ali su se danas pomirili.
(Subject is understood; se is not dropped.)

Why is the order su se and not se su, or su jučer se?

Both su and se are clitics (short unstressed words), and Croatian has strict rules for their order.

Clitics want to stand in the so‑called second position in the clause.
When several clitics appear together, their internal order is fixed.

In this combination, the auxiliary su comes before the reflexive se:

Brat i sestra su se jučer posvađali. – correct
Brat i sestra se su jučer posvađali. – wrong
Brat i sestra su jučer se posvađali. – wrong (you can’t split su and se with jučer)

The block su se must stay together, in that order, immediately after the first “host” in the clause (here: Brat i sestra, and after ali in the second clause).

Can we move jučer and danas to other positions in the sentence, for example to the beginning?

Yes. Jučer and danas are adverbs of time, and Croatian word order allows some flexibility. You just have to keep the clitics (su se) in a correct position.

All of these are possible:

Brat i sestra su se jučer posvađali, ali su se danas pomirili. – neutral order
Jučer su se brat i sestra posvađali, ali su se danas pomirili. – emphasizes “yesterday”
Brat i sestra su se posvađali jučer, ali su se pomirili danas. – adverbs at the end

What you cannot do is violate clitic order or split su and se, for example:

Brat i sestra su jučer se posvađali. – incorrect
Jučer brat i sestra su se posvađali. – sounds strange/ungrammatical in standard Croatian; su se wants to be earlier.

Why is the auxiliary su repeated in the second clause? Could we say Brat i sestra su se jučer posvađali, ali se danas pomirili?

In standard Croatian, each clause normally needs its own finite verb (here: its own auxiliary su).

So we have two full clauses:

Brat i sestra su se jučer posvađali
(Brat i sestra) su se danas pomirili

→ joined by ali.

If you say:

… ali se danas pomirili.

then the second clause has no finite verb (se pomirili is only participle + clitic, but no present‑tense auxiliary). That sounds incomplete or dialectal.

In careful/standard language, you repeat the auxiliary:

… ali su se danas pomirili. – recommended

Why is there a comma before ali?

Because ali connects two independent clauses, each with its own verb:

Clause 1: Brat i sestra su se jučer posvađali
Clause 2: (Brat i sestra) su se danas pomirili

In standard Croatian punctuation, when ali (but) joins two clauses like this, you put a comma before it:

…, ali …

You would not normally omit the comma in this type of sentence.

Why do we use i between brat and sestra, not a?

Both i and a can sometimes be translated as and, but they are used differently:

i – neutral and, simply adds or lists things
a – and/but, often marks a contrast or a slight opposition

Here we just have a neutral pair:

brat i sestra – brother and sister (as one unit)

Using a would suggest some kind of contrast between brat and sestra, which is not intended here and sounds unnatural:

?brat a sestra – would feel odd in this role

So the normal, idiomatic form is brat i sestra.

What case are brat and sestra in here, and why?

They are in the nominative singular:

brat – nominative singular masculine
sestra – nominative singular feminine

Reason: they form the subject of the sentence – the people who performed the actions (quarreled, reconciled). In Croatian, subjects are in the nominative case.

For comparison:

Vidim brata i sestru. – I see (my) brother and sister.
- brata, sestru are now in the accusative, because they are direct objects, not the subject.

What exactly are posvađali and pomirili as grammatical forms?

They are:

active past participles (Croatian: glagolski pridjev radni),
in the masculine plural form.

From the infinitives:

posvađati se →
- posvađao (m.sg.), posvađala (f.sg.), posvađali (m.pl./mixed), etc.
pomiriti se →
- pomirio (m.sg.), pomirila (f.sg.), pomirili (m.pl./mixed), etc.

Combined with the auxiliary biti in the present tense, they make the past tense:

oni su se posvađali
oni su se pomirili

What is the difference between svađati se and posvađati se, and between miriti se and pomiriti se?

This is about aspect (imperfective vs perfective).

svađati se – imperfective
- to be arguing / to argue (as an ongoing or repeated action)
- focuses on the process
posvađati se – perfective
- to (end up) having a fight, to quarrel (as a single completed event)
- focuses on the result / completed event

Similarly:

miriti se – imperfective
- to be making up, to be reconciling
pomiriti se – perfective
- to (finally) make up, to reconcile (completed)

In the sentence:

su se jučer posvađali – they (ended up) having a quarrel yesterday (one completed event)
su se danas pomirili – they (ended up) making up today

If you used imperfective forms, it would suggest a more ongoing or habitual action.

Can we say Brat i sestra su se jučer svađali, ali su se danas pomirili instead? Does it change the meaning?

Yes, that sentence is grammatically correct, but the nuance changes.

su se jučer svađali – they were arguing yesterday (emphasis on the ongoing activity, maybe for some time)
su se jučer posvađali – they (ended up) having a fight yesterday (focus on the completed event; we think of one quarrel as a whole)

So:

svađali su se jučer – more about the process of arguing
posvađali su se jučer – more about the fact that a quarrel happened and was completed

The second part su se danas pomirili keeps the idea that today they finally made up.

What kind of words are jučer and danas? Do they change form for case, gender, or number?

Jučer and danas are adverbs of time:

jučer – yesterday
danas – today

As adverbs, they are invariable:

they do not change for case, gender, or number
you always say jučer, danas, regardless of who you are talking about or what case other words are in

Only their position in the sentence can change (for emphasis, rhythm), but their form stays the same.