Probno razdoblje počinje danas.

Because probno has to agree with razdoblje in gender, number, and case.

• razdoblje is a neuter singular noun
• the adjective therefore also has to be neuter singular
• that gives probno

So:

• probni = masculine
• probna = feminine
• probno = neuter

Since razdoblje is neuter, probno razdoblje is the correct match.

It is in the nominative singular.

Here it is the subject of the sentence, so nominative is exactly what you would expect.

A quick breakdown:

• probno razdoblje = nominative singular subject
• počinje = verb
• danas = adverb of time, so it does not take a case here

Počinje is 3rd person singular present tense.

It matches the subject probno razdoblje, which is singular.

So the pattern is:

• ja počinjem = I begin / am beginning
• ti počinješ = you begin
• on/ona/ono počinje = he/she/it begins

In this sentence, počinje means begins / is starting.

Because Croatian, like English, often uses the present tense for something that starts now or at a scheduled time.

So Probno razdoblje počinje danas is very natural, just like English The trial period starts today.

It does not have to mean that the action is already in progress at this exact second. It can also mean that today is the starting date.

The form počinje comes from the infinitive počinjati.

This is related to the verb pair:

• počinjati = imperfective
• početi = perfective

That is why you may also see:

• počinje from počinjati
• počne from početi

Both can relate to beginning, but the nuance is a little different.

Both are possible in some contexts, but they are not exactly the same.

• počinje comes from the imperfective verb počinjati
• počne comes from the perfective verb početi

In a sentence like this, počinje danas sounds very natural for a scheduled or announced start:

• Probno razdoblje počinje danas.

Počne danas is also understandable, but it can sound a bit more like a single completed starting event rather than a neutral statement about when the period begins.

For many learners, it is safest to remember:

• počinje danas = very natural for starts today

In the present tense, Croatian verbs agree mainly with person and number, not with gender.

So počinje is the same for:

• on počinje = he begins
• ona počinje = she begins
• ono počinje = it begins

Gender becomes more visible in the past tense:

• on je počeo
• ona je počela
• ono je počelo

So in your sentence, the verb does not change just because razdoblje is neuter.

Because Croatian has no articles.

There is no direct equivalent of English a or the in ordinary usage. Context tells you whether something is definite or indefinite.

So probno razdoblje can mean:

• a trial period
• the trial period

In this sentence, the meaning is understood from context, not from an article.

Yes. Croatian word order is more flexible than English word order.

The sentence you have is perfectly natural:

• Probno razdoblje počinje danas.

But you could also say:

• Danas počinje probno razdoblje.

That version puts more emphasis on today.

In general:

• sentence-initial position often gives emphasis or topical focus
• the basic meaning stays the same

So both are correct; the choice depends on what you want to highlight.

A rough pronunciation guide:

• probno ≈ PROB-no
• razdoblje ≈ RAZ-dob-lye
• počinje ≈ po-CHEEN-ye
• danas ≈ DAH-nas

A few important sounds:

• č sounds like ch in church
• nj sounds like ny in canyon
• lj is similar to the lli sound some speakers use in million
• Croatian r is usually trilled or tapped more than in English

So počinje has that č + nj combination that English speakers often need a little practice with.

Yes, probno razdoblje is a normal and standard way to say trial period.

Depending on context, Croatian may also use other expressions, for example:

• probni rok = often probation period, especially in work or legal contexts
• period = also used, but often sounds more international or less specifically standard in some contexts

So:

• probno razdoblje = very good, neutral, standard
• probni rok = also very common, especially for employment probation

Both are useful to know.