Specijalistica je objasnila da injekcija nije bolna i da se ne trebamo bojati.

Because specijalistica is the feminine form, meaning a female specialist. Croatian often marks profession nouns for gender.

• specijalist = a male specialist
• specijalistica = a female specialist

Since the sentence uses objasnila, which is also feminine, we know the specialist is female.

Je objasnila is the Croatian perfect tense, which is the normal way to talk about a completed past action.

It is built from:

• je = present tense of biti in the 3rd person singular
• objasnila = the past participle

So je objasnila means explained / has explained, depending on context.

In Croatian, the past participle agrees with the subject in gender and number.

Here the subject is specijalistica, which is feminine singular, so the participle is:

• objasnila = feminine singular

Compare:

• objasnio = masculine singular
• objasnila = feminine singular
• objasnili = masculine/mixed plural
• objasnije? no — that form does not exist here
• objasnile = feminine plural

So objasnila tells you that the person who explained was female.

Here da introduces a subordinate clause and usually corresponds to English that.

So:

• objasnila da... = explained that...

Croatian uses da very often after verbs like say, explain, know, think, and similar verbs.

Because there are two coordinated subordinate clauses, and each one is introduced by da:

• da injekcija nije bolna
• i da se ne trebamo bojati

Repeating da is very normal and clear in Croatian. It helps show that both parts depend on objasnila.

In informal speech, people may sometimes omit the second da, but repeating it is the standard and safest choice.

Injekcija is in the nominative singular because it is the subject of the clause:

• injekcija nije bolna = the injection is not painful

You can tell it is nominative because it is the thing being described by nije bolna.

Because bolna is an adjective and it must agree with injekcija.

Since injekcija is:

• feminine
• singular
• nominative

the adjective must also be:

• feminine
• singular
• nominative

So:

• bolan = masculine
• bolna = feminine
• bolno = neuter

That is why nije bolna is the correct form.

Nije is the negative form of je in the 3rd person singular.

So:

• je = is
• nije = is not

In other words:

• injekcija je bolna = the injection is painful
• injekcija nije bolna = the injection is not painful

Because Croatian usually leaves subject pronouns out when the verb ending already shows the person.

The ending -mo in trebamo already means we.

So:

• trebamo = we need / we should
• mi trebamo = we need / we should, but with extra emphasis or contrast

Croatian is a language where subject pronouns are often omitted unless they are needed for stress, contrast, or clarity.

Trebamo is the 1st person plural present tense of trebati.

Here it is followed by an infinitive:

• trebamo bojati se in basic dictionary-style logic
• actual sentence order: se ne trebamo bojati

With an infinitive, trebati often means:

• to need to
• to be supposed to
• should

So ne trebamo bojati se here means something like:

• we do not need to be afraid
• we should not be afraid

The exact English wording depends on context, but the idea is reassurance.

This is mainly because of clitic placement.

The word se is a clitic, and Croatian clitics usually appear very early in the clause. In a clause introduced by da, se commonly comes right after da:

• da se...

Then ne goes directly before the finite verb:

• ne trebamo

So the order becomes:

• da se ne trebamo bojati

Not:

• da ne se trebamo bojati ← incorrect

A useful comparison:

• independent sentence: Ne trebamo se bojati.
• subordinate clause: ...da se ne trebamo bojati.

Yes. The dictionary form is bojati se, meaning to be afraid.

The se is part of the verb’s normal form, so you should learn it together:

• bojati se = to be afraid
• bojim se = I am afraid
• bojimo se = we are afraid

When this verb has an object, that object is often in the genitive:

• Bojim se psa. = I am afraid of the dog.
• Bojimo se problema. = We are afraid of problems.

In your sentence, there is no object after bojati se.

Yes, but the meaning would change.

• da se ne bojimo = that we are not afraid
• da se ne trebamo bojati = that we do not need to be afraid / that we should not be afraid

So ne bojimo se describes an actual emotional state, while ne trebamo se bojati gives advice or reassurance.

It comes from the perfective verb objasniti.

That means the action is seen as completed: she finished explaining.

Compare:

• objasniti → perfective
• objašnjavati → imperfective

So:

• Specijalistica je objasnila... = she explained it, as a completed act
• Specijalistica je objašnjavala... = she was explaining / used to explain

This is a common Croatian aspect contrast.

Because i normally does not take a comma when it simply joins two parallel elements, including two subordinate clauses like these:

• da injekcija nije bolna
• i da se ne trebamo bojati

So the structure is treated as one smooth coordination, and no comma is needed before i.