Majstor kaže da je osigurač star, ali da je aparat još dobar i da ga ne trebamo mijenjati.

Croatian often repeats da before each new reported clause after a verb like kaže.

Here the sentence breaks into three parts:

• da je osigurač star
• ali da je aparat još dobar
• i da ga ne trebamo mijenjati

In English, you might say The technician says the fuse is old, but the appliance is still good and we don't need to replace it, without repeating that. In Croatian, repeating da is very natural.

Because je is a clitic form of biti (to be). Croatian clitics usually go in second position in their clause.

So after da, the clitic comes immediately:

• da je osigurač star
• da je aparat još dobar

This is standard Croatian word order.

Because here the adjectives are used after the verb je, as predicate adjectives:

• osigurač je star
• aparat je dobar

In that position, Croatian normally uses star, dobar, nov, loš, and so on.

Forms like stari and dobri are usually used before a noun:

• stari osigurač = an/the old fuse
• dobar aparat = a good appliance

Here još means still.

So:

• aparat je još dobar = the appliance is still good

In other contexts, još can also mean more, another, or yet, so context matters.

Ga is an unstressed pronoun meaning him or it for a masculine singular noun. Here it means it.

It most naturally refers to aparat, because the sentence says the appliance is still good and therefore does not need replacing. Grammatically, it could also match osigurač, since osigurač is masculine too, but the meaning strongly points to aparat.

Because ga is also a clitic, and Croatian clitics usually appear very early in the clause.

After da, clitics tend to come right away:

• da ga ne trebamo mijenjati

This is much more natural than:

• da ne trebamo ga mijenjati

So the short pattern is:

• da + clitic + verb + rest

It is built into the verb ending. Trebamo means we need or we need to. The ending -mo marks first person plural.

So Croatian does not need a separate word for we here. You could add mi for emphasis, but it is not necessary:

• Mi ga ne trebamo mijenjati.

Here trebati means to need to or to be necessary to:

• ne trebamo mijenjati = we do not need to replace/change

Morati is stronger and means must / have to.

Compare:

• Ne trebamo ga mijenjati = We don't need to replace it
• Ne moramo ga mijenjati = We don't have to replace it

In many situations they are close, but trebati sounds more like it isn't necessary.

This is about aspect.

• mijenjati is imperfective
• zamijeniti is perfective

In repair contexts, mijenjati can mean to replace/change in a general sense. After a verb like trebati, Croatian often uses the imperfective when talking about whether an action is necessary at all.

So:

• ga ne trebamo mijenjati = there is no need to replace it

If you used zamijeniti, the idea would be more like carry out one completed replacement. That can be possible in some contexts, but mijenjati is the more neutral choice here.

Yes. The main clause is:

• Majstor kaže = The technician says

Then come three linked subordinate clauses:

• da je osigurač star
• ali da je aparat još dobar
• i da ga ne trebamo mijenjati

So the sentence is basically reporting three pieces of information in one structure.