Apsolutno želim posijati bosiljak.

Yes, it is natural. Apsolutno adds strong emphasis, something like absolutely, definitely, or really.

So the sentence is not just a neutral I want to sow basil, but a more emphatic I absolutely / definitely want to sow basil.

A few notes:

• It sounds stronger than plain želim posijati bosiljak.
• In everyday speech, people might also use stvarno, baš, or definitivno, depending on the tone they want.

Želim is the 1st person singular present tense of željeti, which means to want.

So:

• želim = I want
• želiš = you want
• želi = he/she/it wants

In this sentence, želim matches the understood subject I.

Because after želim in standard Croatian, you normally use an infinitive to express what someone wants to do.

So:

• želim posijati = I want to sow

This works much like English want to do something, except Croatian does not use a separate word like to before the infinitive. The infinitive itself is the form posijati.

This is a question about aspect, which is very important in Croatian.

• sijati = imperfective
• posijati = perfective

Very roughly:

• sijati focuses on the process, repetition, or general activity of sowing
• posijati focuses on a single completed act of sowing

In this sentence, posijati is natural because the speaker means a definite, completed action: they want to actually get the basil sown.

Compare:

• Želim sijati bosiljak. = I want to be sowing / sow basil in general
• Želim posijati bosiljak. = I want to sow basil, as a completed action

Because posijati means to sow seeds, while posaditi means to plant something such as a seedling, young plant, tree, or flower.

So:

• posijati bosiljak = sow basil seeds
• posaditi bosiljak = plant basil, for example a small basil plant you bought somewhere

If the idea is starting basil from seed, posijati is exactly the right verb.

Bosiljak is the direct object, so grammatically it is in the accusative case.

However, for masculine inanimate singular nouns, the accusative is often the same as the nominative. That is why bosiljak does not visibly change here.

So:

• nominative: bosiljak
• accusative: bosiljak

That is normal.

For comparison, masculine animate nouns usually do change:

• pas = dog
• vidim psa = I see the dog

But with bosiljak, which is inanimate, the form stays the same.

People will understand it, but in standard Croatian, želim posijati bosiljak with the infinitive is preferred.

So:

• more standard Croatian: Želim posijati bosiljak
• understandable, but less standard in Croatian: Želim da posijem bosiljak

The da + present tense structure is much more typical in some neighboring standard varieties and in colloquial speech. If you want to sound solidly standard Croatian, use the infinitive after želim.

No, Croatian word order is fairly flexible, but different orders create different emphasis.

Your sentence:

• Apsolutno želim posijati bosiljak.

This sounds natural and puts emphasis on the speaker’s strong certainty or determination.

Other possible orders include:

• Želim posijati bosiljak. = neutral
• Bosiljak želim posijati. = emphasizes basil specifically, maybe contrasting it with something else
• Želim bosiljak posijati. = possible, but less neutral in tone

So the original order is good and natural if the point is I really do want to do this.

A rough English-friendly guide would be:

• Apsolutno ≈ ap-so-LOOT-no
• želim ≈ ZHE-lim
• posijati ≈ po-see-YA-ti
• bosiljak ≈ bo-SIL-yak

Some important sounds:

• ž sounds like the s in measure
• lj is a single sound, roughly similar to the lli in some pronunciations of million
• in posijati, the ij is pronounced clearly, so it sounds roughly like see-ya in the middle

One helpful thing about Croatian is that spelling is much more phonetic than English spelling.

Because Croatian does not have articles like a, an, or the.

So bosiljak can mean:

• basil in general
• some basil
• the basil

The exact meaning depends on context.

That is completely normal in Croatian. You do not need to add anything where English would use a or the.

It can mean both, depending on context.

For example, bosiljak may refer to:

• the basil plant
• basil as an herb
• basil in a general sense

In this sentence, because the verb is posijati, the most natural interpretation is basil as something being grown from seed. So the focus is on the plant/crop rather than basil as a spice sitting in a jar.