Začiniću marinadu začinom.

It is the 1st person singular future form of začiniti.

You can express this future in two common ways:

• Ja ću začiniti marinadu.
• Začiniću marinadu.

In the second version, the auxiliary ću is attached to the verb, so it becomes one written word.

A useful way to think about it is:

• infinitive: začiniti
• future auxiliary: ću
• combined form: začiniću

This verb is also perfective, so it suggests a completed action: I will season it / I’ll add seasoning to it.

Because marinadu is the accusative singular form of marinada.

Here, marinadu is the direct object of the verb začiniću. In Serbian, the direct object often goes in the accusative case.

So:

• nominative: marinada
• accusative: marinadu

That is why the sentence uses marinadu.

Začinom is the instrumental singular form of začin.

The instrumental case here shows the means or thing used to do the action:

• začiniti marinadu začinom = to season the marinade with seasoning / with a spice

So the forms are:

• nominative: začin
• instrumental: začinom

The ending -om is very common for masculine singular nouns in the instrumental.

Because Serbian often uses the instrumental case without a preposition to express the idea of with, especially when it means by means of or using something.

So:

• začinom can mean with a spice / with seasoning
• you do not need a separate word like English with

In other words, the case ending itself carries that meaning.

You may also see sa or s with instrumental in other contexts, but here the bare instrumental is perfectly normal.

Yes. They are closely related.

• začin = spice, seasoning
• začiniti = to season, to spice

So the sentence has a repeated root:

• Začiniću marinadu začinom.

That makes it very easy to see the connection, but it can also sound a little repetitive.

It is grammatical, but it does sound somewhat repetitive because of the repeated začin- root:

• začiniću
• začinom

A native speaker would understand it immediately, but in everyday speech they might prefer something a bit more specific, such as:

• Začiniću marinadu ovim začinom.
• Marinadu ću začiniti začinima.
• Dobro ću začiniti marinadu.

So the original sentence is fine as a grammar example, but in real conversation you might hear a less repetitive version.

Because Serbian usually does not need the subject pronoun when the verb already shows who is doing the action.

In začiniću, the ending tells you the subject is I.

So:

• Začiniću marinadu začinom.
• Ja ću začiniti marinadu.

Both mean the same thing, but the first one omits ja because it is already understood.

You would add ja only if you want emphasis or contrast, for example:

• Ja ću začiniti marinadu, ne ti.

Yes. Serbian word order is fairly flexible because the case endings show the grammatical roles.

For example, all of these are possible:

• Začiniću marinadu začinom.
• Marinadu ću začiniti začinom.
• Začinom ću začiniti marinadu.

They all express basically the same idea, but the emphasis changes:

• starting with marinadu puts more focus on the marinade
• starting with začinom puts more focus on the spice/seasoning

One important detail: if ću is written separately, it usually goes in second position in the sentence or clause.

The verb začiniti is perfective, so it presents the action as a single completed whole.

So the sentence suggests:

• I will season the marinade
  rather than
• I will be seasoning the marinade or I season marinade regularly

This is very common in Serbian. A perfective verb in the future often means a one-time completed future action.

This sentence is useful because it contains both sounds.

• č in začin- is a harder sound, roughly like ch in church
• ć in ću is softer; English does not have an exact equivalent, but learners often approximate it as a very soft ty/ch sound

So:

• začin starts with the harder č
• ću has the softer ć

This difference matters in Serbian, because č and ć are different letters and different sounds.

Serbian has no articles like English a and the, so the sentence itself does not tell you that directly.

Depending on context:

• marinadu can mean the marinade or a marinade
• začinom can mean with a spice, with seasoning, or with the spice

Context decides which one is meant.

That is very normal in Serbian, and learners usually get used to it quickly.