On je htio sjediti blizu glazbe, ali je umjesto toga sjeo pokraj djeda i mirno razgovarao.

Both are possible.

• Htio je = He wanted
• On je htio = He wanted

The pronoun on is often optional in Croatian because the verb form already shows the person. Here, on is included for emphasis, contrast, or simply because the speaker chose to make the subject explicit.

Also, je is a clitic, so it tends to appear in the second position of the clause. That is why you get:

• On je htio
• not normally On htio je

Htio is the masculine singular past participle of htjeti, meaning to want.

So:

• htjeti = to want
• htio je = he wanted

Because the subject is masculine singular (on), the form is htio.

Compare:

• On je htio = he wanted
• Ona je htjela = she wanted
• Oni su htjeli = they wanted

Yes, but they express two different ideas.

• sjediti = to be sitting / to sit as a state or ongoing action
• sjesti = to sit down as a completed action of taking a seat

In the sentence:

• htio sjediti blizu glazbe = he wanted to sit / be sitting near the music
• sjeo pokraj djeda = he sat down next to grandpa

This is a very common Croatian aspect distinction:

• sjediti = imperfective, ongoing state
• sjesti / sjeo = perfective, completed action

In English, both may involve sit, but Croatian separates being seated from the act of sitting down more clearly.

Because sjeo comes from sjesti and means sat down, while sjedio comes from sjediti and means was sitting / sat in the sense of being seated.

So:

• sjeo = sat down
• sjedio = was sitting

Here the sentence describes the action of moving into a seat next to grandpa, so sjeo is the right choice.

Because blizu normally takes the genitive case.

So:

• glazba = music
• glazbe = of music / music in the genitive form

That is why Croatian says:

• blizu glazbe = near the music

The same pattern appears with many expressions of nearness or position:

• blizu kuće = near the house
• blizu škole = near the school

Pokraj also takes the genitive case.

So:

• djed = grandfather / grandpa
• djeda = grandfather in the genitive singular

Therefore:

• pokraj djeda = next to grandpa

This is similar to:

• pokraj kuće = next to the house
• pokraj prozora = next to the window

They are related, but not identical.

• blizu = near, close to
• pokraj = beside, next to

So in this sentence:

• blizu glazbe = close to the music
• pokraj djeda = next to grandpa

Blizu is a bit less exact: something is nearby. Pokraj usually suggests being directly beside something or someone.

It means instead of that or more naturally instead.

Word by word:

• umjesto = instead of
• toga = that, in the genitive

Together:

• umjesto toga = instead of that / instead

In this sentence, it refers back to the earlier idea:

• he wanted to sit near the music,
• but instead he sat next to grandpa

Because umjesto requires the genitive case.

The pronoun to changes in the genitive singular to toga.

So:

• to = that
• toga = of that / that in the genitive

This is why Croatian says:

• umjesto toga = instead of that

Because je is the auxiliary used to form the past tense, and Croatian clitics usually go in the second position of the clause.

So in the new clause:

• ali = but
• je = auxiliary
• umjesto toga sjeo = sat down instead

The word ali counts as the first element, so the clitic je comes right after it:

• ali je sjeo...

This is very typical Croatian word order.

Because Croatian often omits the repeated auxiliary when two past-tense verbs are joined and share the same subject.

Full version:

• ...ali je umjesto toga sjeo pokraj djeda i mirno je razgovarao.

More natural shortened version:

• ...ali je umjesto toga sjeo pokraj djeda i mirno razgovarao.

The second je is understood from the first one. This is very common and sounds natural.

Here mirno is an adverb, not an adjective.

• miran = calm, peaceful
• mirno = calmly, quietly, peacefully

So:

• mirno razgovarao = talked calmly / spoke quietly

The adverb describes how he talked.

Compare:

• miran čovjek = a calm man
• govorio je mirno = he spoke calmly

In standard Croatian, glazba is the normal word for music.

• glazba = standard Croatian
• muzika = understood, but less standard in Croatian and more associated with other regional varieties or informal usage

So in a standard Croatian sentence, blizu glazbe is the expected choice.

It could, in many contexts.

• kraj djeda
• pokraj djeda

Both can mean next to grandpa. In many situations they are very close in meaning.

However:

• pokraj often feels a bit more explicitly spatial, like beside
• kraj can also mean by, beside, or appear in other uses depending on context

In this sentence, pokraj djeda is perfectly natural and clearly means he sat beside him.

The main actions are in the past tense, specifically the Croatian perfect:

• je htio = wanted
• je sjeo = sat down
• razgovarao = talked, with the auxiliary understood

Croatian perfect is usually formed with:

• present tense of biti as an auxiliary
• plus the past participle

Examples from the sentence:

• je htio
• je sjeo
• (je) razgovarao

Because Croatian commonly uses the infinitive directly after verbs like want, can, must, and similar verbs.

So:

• htio sjediti = wanted to sit
• literally: wanted sit

English needs to, but Croatian does not here.

Other examples:

• Želim jesti. = I want to eat.
• Mogu doći. = I can come.
• Moram raditi. = I must work.

Yes, Croatian word order is fairly flexible, though some versions sound more natural than others.

For example, these are possible with different emphasis:

• On je htio sjediti blizu glazbe, ali je umjesto toga sjeo pokraj djeda i mirno razgovarao.
• Htio je sjediti blizu glazbe, ali je umjesto toga sjeo pokraj djeda i mirno razgovarao.
• On je htio sjediti blizu glazbe, ali je sjeo pokraj djeda i umjesto toga mirno razgovarao.
  This shifts the emphasis somewhat and may sound less natural depending on the intended meaning.

Even with flexible word order, clitics like je still have strong placement rules, so not every rearrangement works.