Ona mi je objasnila gde je greška u poruci.

Yes, ona can often be omitted in Serbian because the verb form already shows person and number.

• Ona mi je objasnila... = She explained to me...
• Mi je objasnila... would still be understandable from the verb objasnila that the subject is feminine singular: she.

However, ona is often kept for:

• emphasis,
• contrast,
• clarity.

So this sentence without ona is also natural:

Objasnila mi je gde je greška u poruci.

That still means the same thing.

Mi here means to me.

It is the unstressed dative form of ja:

• ja = I
• meni / mi = to me

So:

• Ona mi je objasnila... = She explained to me...

This is very common with verbs like:

• reći = to tell
• dati = to give
• pokazati = to show
• objasniti / objašnjavati = to explain

You could also say meni, but that is the stressed form and is usually used for emphasis:

• Ona je meni objasnila... = She explained it to me (maybe not to someone else)

Yes. Je objasnila is part of the Serbian past tense.

Serbian past tense is usually formed with:

1. the present tense of biti (sam, si, je, smo, ste, su),
2. plus the past participle.

Here:

• je = is/has, the auxiliary for she
• objasnila = past participle, feminine singular

So:

• ona je objasnila = she explained / she has explained

In natural English, this is usually just translated as she explained.

Because the subject is ona, which is feminine singular.

In Serbian, the past participle agrees with the subject in gender and number:

• on je objasnio = he explained
• ona je objasnila = she explained
• ono je objasnilo = it explained
• oni su objasnili = they explained (masculine/mixed)
• one su objasnile = they explained (all feminine)

So objasnila must match ona.

Because Serbian has special unstressed words called clitics, and they usually go in a fixed position near the beginning of the clause.

In this sentence:

• mi = unstressed dative pronoun
• je = unstressed auxiliary

These clitics usually come in a set order, and mi comes before je.

So:

• Ona mi je objasnila... = correct
• Ona je mi objasnila... = not correct in standard Serbian

This clitic order is something learners gradually get used to. You will see similar patterns often:

• Dao mi je knjigu.
• Rekla mu je istinu.
• Pokazali su nam put.

It looks like a question, but here it is actually an embedded question or indirect question.

Literally:

• gde je greška = where the mistake is

So the whole sentence means something like:

• She explained to me where the mistake is in the message.

Compare:

Direct question:

• Gde je greška? = Where is the mistake?

Embedded question:

• Objasnila mi je gde je greška. = She explained to me where the mistake is.

Serbian does not need a word like whether or a special word order here. It keeps the normal clause order.

Because greška is the subject of the embedded clause gde je greška.

In that clause:

• greška = the mistake/error
• je = is

So greška stays in the nominative, because it is the thing that is somewhere.

Think of it as:

• Greška je u poruci. = The mistake is in the message.

Then that whole idea is inserted into the bigger sentence:

• Ona mi je objasnila [gde je greška u poruci].

Poruci is in the locative case after the preposition u when it means in a location or context.

• poruka = message
• u poruci = in the message

So:

• greška u poruci = the mistake in the message

This is different from u + accusative, which often shows motion toward something:

• u poruci = in the message
• u poruku = into the message (much less likely in this context)

Here there is no movement, just location, so locative is used.

Poruka is a general word for message. It can mean:

• a text message,
• an email message,
• a written message,
• a spoken message,
• a general communication.

So u poruci could mean:

• in the text message,
• in the email,
• in the message.

The exact meaning depends on context.

Both forms exist, depending on the regional standard and pronunciation system.

• gde is standard in Serbian Ekavian
• gdje is used in ijekavian varieties

Since this sentence is in standard Serbian Ekavian, gde is completely normal.

You may also see:

• ovde / ovdje = here
• negde / negdje = somewhere

So this is mostly a variety/spelling/pronunciation difference, not a difference in meaning.

Not exactly. The difference is mainly aspect.

• objasniti (perfective) = to explain, in the sense of completing the explanation
• objašnjavati (imperfective) = to be explaining, to explain repeatedly, to explain in general

So:

• Ona mi je objasnila gde je greška u poruci. = She explained to me where the mistake is in the message. = the explanation is treated as completed

• Ona mi je objašnjavala gde je greška u poruci. = She was explaining to me where the mistake is / where the mistake was. = focuses more on the process, not necessarily completion

This perfective/imperfective distinction is very important in Serbian.

Yes, Serbian word order is fairly flexible, although some orders sound more natural than others because of clitics and emphasis.

Neutral and very natural:

• Ona mi je objasnila gde je greška u poruci.

Also natural:

• Objasnila mi je gde je greška u poruci.
• Gde je greška u poruci, ona mi je objasnila. (more marked, emphasizing the content)
• U poruci mi je objasnila gde je greška. (changes emphasis and may sound less neutral depending on context)

So the basic meaning can stay the same, but the focus or style may change.

The Serbian sentence literally has present tense inside the embedded clause:

• gde je greška = where the mistake is

That is perfectly normal in Serbian if the mistake still exists in the message now.

In English, depending on context, you might say:

• She explained where the mistake is in the message.
• She explained where the mistake was in the message.

If the speaker is talking about a specific past situation, English often prefers was. Serbian can still use je if the mistake is understood as still being there in the message. So the best English translation depends on context, even though the Serbian form is clear and natural.