Na početku je bilo dogovoreno da radimo u parovima, ali je na kraju odlučeno da zadatak riješimo sami.

They are both in the locative singular:

• početak → početku
• kraj → kraju

That is because Croatian uses na + locative in expressions like:

• na početku = at the beginning
• na kraju = at the end / in the end

So these are very common time expressions, even though they literally look a bit like location phrases.

This is an impersonal passive-style construction, very common in Croatian.

• je bilo dogovoreno = it had been agreed / it was agreed
• je odlučeno = it was decided

English often uses a dummy subject, it, but Croatian usually does not need an equivalent subject here.

Also, dogovoreno and odlučeno are in neuter singular, which is the default form when there is no real subject named.

So Croatian is focusing on the fact that an agreement/decision existed, not on the people who made it.

The form je bilo dogovoreno suggests that the agreement was already in place earlier, before the later change. It gives a sense of an earlier existing arrangement.

So the contrast is roughly:

• at first, it had been agreed
• in the end, it was decided differently

In other words, bilo helps make the first clause feel more clearly prior to the second one.

The second clause does not need bilo, because it presents the later decision directly.

Because after da, Croatian very often uses a present-tense form to express something that was planned, intended, or decided.

So here:

• da radimo u parovima
• da zadatak riješimo sami

do not mean present time in the usual sense. They mean something like:

• that we should work in pairs
• that we should solve the task ourselves

This is normal Croatian grammar. The time reference comes from the main verbs dogovoreno and odlučeno, not from the present forms themselves.

This is a matter of aspect, which is very important in Croatian.

• raditi is imperfective: it describes an ongoing activity, process, or repeated action.
• riješiti is perfective: it describes reaching a result, completing something.

So:

• radimo u parovima focuses on the activity or mode of working
• zadatak riješimo sami focuses on completing the task

That is very natural here. You work in pairs as a process, but you solve a task as a completed result.

U parovima is locative plural.

• singular: par
• locative plural: parovima

After u, Croatian often uses the locative when it means being or happening in something.

Here u parovima means in pairs.

It is plural because the idea is that several people are divided into multiple pairs. If you were talking about just one pair, you could say u paru.

Here sami means by ourselves, on our own, or without partners.

It agrees with the understood subject mi = we.

So the idea is:

• first: work in pairs
• later: solve the task ourselves

It does not necessarily mean physically alone; it means doing it without working with a partner.

Because it agrees with the implied subject we.

The hidden subject of both subordinate clauses is mi:

• da radimo
• da riješimo sami

For a mixed group or a general we, Croatian uses sami.
If the group were specifically all female, you would expect same instead.

So sami is the normal default form here.

In this sentence, da + present is the natural choice.

After impersonal expressions like:

• dogovoreno je
• odlučeno je

Croatian normally states the content with a da-clause:

• dogovoreno da radimo...
• odlučeno da riješimo...

If you wanted an infinitive, you would usually change the structure and make the subject explicit, for example:

• Odlučili smo riješiti zadatak sami.

So the infinitive is possible in Croatian in general, but not as naturally in this exact structure.

Because je is a clitic, and Croatian clitics usually stand very early in the clause.

That is why Croatian often places je, sam, se, ga, and similar short forms near the beginning.

So:

• ali je na kraju odlučeno

sounds very natural.

Croatian word order is flexible, but clitics are one of the main things that limit that flexibility. Learners often notice them because they do not behave like full words in English.