S roditeljima se lakše dogovaram nego s bratom, jer on često mijenja planove u zadnji čas.

Both roditeljima and bratom are in the instrumental case because they follow the preposition s meaning with.

• roditelji = parents
• s roditeljima = with the parents / with my parents
• brat = brother
• s bratom = with my brother

In Croatian, s + instrumental is the normal way to say with someone.

The verb here is dogovarati se, not just dogovarati.

dogovarati se means:

• to make arrangements
• to come to an agreement
• to coordinate / agree on plans

The se is part of the verb and is very important. Without it, the meaning changes or becomes unnatural in this context.

So:

• dogovaram se s roditeljima = I make arrangements / reach agreement with my parents
• lakše se dogovaram = I agree more easily / it’s easier for me to arrange things

English learners often want to translate this very literally, but it is better to learn dogovarati se s nekim as a whole pattern:

• dogovarati se s + instrumental

Here lakše is an adverb, meaning more easily.

It comes from:

• lako = easily
• lakše = more easily

So:

• S roditeljima se lakše dogovaram = I come to an agreement more easily with my parents

It is not describing a noun, so it is not an adjective here. It describes how the agreeing happens.

Here nego is used because the sentence compares one situation with another:

• lakše ... nego ... = more easily ... than ...

So:

• S roditeljima se lakše dogovaram nego s bratom
  = I get along / make arrangements more easily with my parents than with my brother

You usually use nego after a comparative in this kind of sentence.

You may also see od in some comparisons, but here nego is the natural choice because you are comparing two full phrases:

• s roditeljima
• s bratom

Croatian often leaves out possessive words like my, your, his, etc. when the meaning is already clear from context.

So:

• s roditeljima can naturally mean with my parents
• s bratom can naturally mean with my brother

If needed, Croatian could say:

• sa svojim roditeljima
• sa svojim bratom

But that would be more explicit, and often unnecessary.

This is an aspect difference.

• dogovarati se = imperfective
• dogovoriti se = perfective

In this sentence, dogovaram se is used because the speaker is talking about a general, repeated situation:

• S roditeljima se lakše dogovaram...
  = I generally have an easier time making arrangements with my parents...

If you said dogovorim se, it would usually point to a completed agreement in a more specific instance.

Very roughly:

• dogovaram se = I am arranging / I usually arrange / I tend to reach agreement
• dogovorim se = I reach an agreement / I manage to agree

Because the subject is I, not parents.

The verb dogovaram means I arrange / I agree / I make plans.

So the structure is:

• (Ja) se dogovaram s roditeljima
  = I make arrangements with my parents

The parents are not the subject; they are part of the phrase with s.

Yes, Croatian word order is flexible.

This sentence starts with S roditeljima to highlight the comparison right away:

• S roditeljima se lakše dogovaram nego s bratom...

You could also say:

• Lakše se dogovaram s roditeljima nego s bratom.

Both are natural. The difference is mostly one of focus and style, not basic meaning.

Croatian often moves phrases around for emphasis.

On means he, referring to brat.

It is not strictly necessary, because the verb already helps identify the subject:

• jer često mijenja planove = because he often changes plans

But adding on makes the contrast clearer and more explicit:

• ... nego s bratom, jer on često mijenja planove...

It can feel a bit like:

• ... than with my brother, because he often changes plans...

So on is used for clarity or emphasis.

Because planove is the direct object of the verb mijenjati = to change.

The direct object here is in the accusative plural:

• singular: plan
• plural nominative: planovi
• plural accusative: planove

So:

• mijenja planove = changes plans

This is a very common pattern:

• čitati knjigu = read a book
• kupiti karte = buy tickets
• mijenjati planove = change plans

U zadnji čas means at the last moment or at the last minute.

It is a fixed, very common expression.

Examples:

• Javio se u zadnji čas. = He contacted me at the last minute.
• Promijenili su odluku u zadnji čas. = They changed the decision at the last moment.

So in your sentence:

• jer on često mijenja planove u zadnji čas
  = because he often changes plans at the last minute

After u, Croatian can take either:

• accusative for movement/direction or for some fixed expressions
• locative for location

In this expression, u zadnji čas is the normal idiomatic form. It is a set phrase meaning at the last moment.

Learners often expect locative after u, but many time expressions and idioms use accusative:

• u ponedjeljak = on Monday
• ujutro = in the morning
• u zadnji čas = at the last minute

So this is something best learned as a whole expression.

Yes, but mostly in pronunciation and ease of speaking.

The basic preposition is s.
It becomes sa before certain sound combinations to make pronunciation easier.

So both of these are normal:

• s bratom
• sa sestrom

With roditeljima, s roditeljima is fine.
With bratom, s bratom is also fine.

A learner should mainly remember:

• use s normally
• use sa when Croatian pronunciation would be awkward with just s

Yes, literally it can mean with the parents, with parents, or with my parents, depending on context.

But in everyday speech, when someone says:

• S roditeljima se lakše dogovaram...

people will usually understand:

• with my parents

Croatian often relies on context rather than repeating possessives.

Yes. The combination of često + imperfective verb (mijenja) shows a habitual action.

• često = often
• mijenja = changes

So:

• on često mijenja planove = he often changes plans

This suggests something he does repeatedly, not just once.

A very literal translation would be something like:

• I agree more easily
• I arrange things more easily
• I come to an agreement more easily

But in natural English, depending on context, you might say:

• It’s easier to make plans with my parents than with my brother
• I can agree with my parents more easily than with my brother
• I coordinate with my parents more easily than with my brother

So the Croatian expression is flexible, and English may translate it in different natural ways.