Susjeda mi ga je posudila jučer, ali joj adapter moram vratiti večeras.

They are short unstressed pronouns, often called clitics.

• mi = to me (dative)
• ga = him / it (here it, accusative masculine singular)
• joj = to her (dative)

So:

• Susjeda mi ga je posudila = The neighbor lent it to me
• joj adapter moram vratiti = I have to return the adapter to her

Yes. Susjeda is female neighbor.

• susjed = male neighbor
• susjeda = female neighbor

English neighbor does not normally show gender, but Croatian often does.

Because the past participle agrees with the subject in gender and number.

The subject is susjeda, which is:

• feminine
• singular

So the verb form is:

• posudila = feminine singular past participle

Compare:

• susjed je posudio = the male neighbor lent
• susjeda je posudila = the female neighbor lent
• susjedi su posudili = the neighbors lent

Several different cases appear here:

• Susjeda = nominative, because it is the subject
• mi = dative, meaning to me
• ga = accusative, the direct object it
• joj = dative, meaning to her
• adapter = accusative, because it is the thing being returned

This is very typical in Croatian: the person receiving something is often in the dative, while the thing itself is in the accusative.

Because adapter is a masculine inanimate noun, and in Croatian those often have the same form in nominative and accusative singular.

So:

• nominative: adapter
• accusative: adapter

That is why it does not visibly change here, even though it is the object of vratiti.

Because Croatian short pronouns and short auxiliary forms follow special clitic word order rules.

In this sentence, mi, ga, and je form a clitic cluster. Croatian does not place these words as freely as English would. Mi ga je is the natural standard order here.

A learner should get used to these common clitic bundles as whole patterns, for example:

• mi je
• mi ga je
• joj ga je
• dao mi ga je

Because joj is also a clitic, and clitics usually appear very early in the clause, often in second position.

So in:

• ali joj adapter moram vratiti večeras

the clitic joj comes immediately after ali.

That is normal Croatian word order. English speakers often expect a fuller phrase order, but Croatian likes to place these short unstressed words near the beginning.

Croatian could use a pronoun there too:

• ali joj ga moram vratiti večeras

That is grammatically fine if the reference is clear.

But repeating adapter can sound clearer and more natural, especially after a first clause that already contains several clitics. Repeating the noun avoids ambiguity and makes the sentence easier to process.

Because after morati (must / have to), Croatian normally uses the infinitive.

So:

• moram vratiti = I must return

This is the standard pattern:

• moram ići = I must go
• moram raditi = I must work
• moram vratiti adapter = I must return the adapter

This is about aspect.

• vratiti = perfective: return it as one completed act
• vraćati = imperfective: be returning / return repeatedly / focus on process

Since the sentence refers to one specific completed return this evening, vratiti is the natural choice.

Likewise, posudila comes from posuditi, which is also perfective and fits a single completed act of lending yesterday.

It can mean either, depending on the structure.

• posuditi nekome nešto = to lend someone something
• posuditi nešto od nekoga = to borrow something from someone

In your sentence:

• Susjeda mi ga je posudila

the structure is someone + dative person + object, so it means She lent it to me.

That is a very common source of confusion for English speakers.

Yes, Croatian word order is fairly flexible, but the focus or emphasis changes, and clitics still have special placement rules.

For example, you could also say:

• Jučer mi ga je susjeda posudila, ali joj adapter moram vratiti večeras.
• Susjeda mi ga je jučer posudila, ali joj večeras moram vratiti adapter.

These all keep roughly the same meaning, but they highlight different parts of the sentence. What you cannot do freely is move clitics around as if they were normal stressed words.

Because Croatian has no articles.

So susjeda can mean:

• a neighbor
• the neighbor
• sometimes even my neighbor

The exact meaning comes from context. If Croatian wants to be more explicit, it can use other words, such as:

• moja susjeda = my neighbor
• ta susjeda = that neighbor / that particular neighbor

But very often nothing extra is needed.