Kurir je postao umoran.

Je is the 3rd person singular present form of biti (to be) and it is used to form the Serbian past tense.

So:

• kurir = courier
• je postao = became / has become
• umoran = tired

Serbian past tense is usually made with:

• a present-tense form of biti
• plus the past active participle

Here, that gives:

• je
  • postao

So Kurir je postao umoran literally has the structure The courier is become tired, but in natural English that means The courier became tired.

Because je is a clitic, and Serbian clitics usually go in the second position of the sentence or clause.

So in:

• Kurir je postao umoran.

the first element is Kurir, and the clitic je comes right after it.

This second-position rule is very common in Serbian. For example:

• Kurir je stigao. = The courier arrived.
• Danas je umoran. = Today he is tired.
• Moj brat je došao. = My brother came.

Even though English learners may expect the verb to stay together, Serbian often places these short helper words in second position.

Postao is the masculine singular past active participle of postati, which means to become.

So the dictionary form is:

• postati = to become

Its past participle changes depending on gender and number:

• postao = masculine singular
• postala = feminine singular
• postalo = neuter singular
• postali = masculine plural / mixed plural
• postale = feminine plural
• postala = neuter plural

Since kurir is a masculine singular noun, the sentence uses postao.

Because postao must agree with the subject, and the subject here is kurir, which is:

• masculine
• singular

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

Compare:

• Kurir je postao umoran. = The courier became tired.
• Kurirka je postala umorna. = The female courier became tired.
• Kuriri su postali umorni. = The couriers became tired.

So postao is used because the subject is one male courier, or a masculine noun.

Umoran is an adjective meaning tired, and it agrees with the subject kurir.

Since kurir is masculine singular, the adjective is also masculine singular:

• umoran = masculine singular
• umorna = feminine singular
• umorno = neuter singular
• umorni = masculine plural
• umorne = feminine plural
• umorna = neuter plural

Examples:

• Kurir je postao umoran.
• Kurirka je postala umorna.
• Dete je postalo umorno.

So the adjective changes just like many Serbian adjectives do.

It is in the nominative, matching the subject.

After postati when you use an adjective, Serbian normally uses the adjective in nominative:

• postati umoran = to become tired
• postati srećan = to become happy
• postati ozbiljan = to become serious

So umoran is not an object here. It is a predicate adjective describing what the subject became.

Kurir is in the nominative singular because it is the subject of the sentence.

In Serbian, the subject is normally in the nominative. Here:

• kurir = the courier
• nominative singular masculine noun

That is why the verb forms and adjective also line up with masculine singular.

Because Serbian has no articles like English a/an/the.

So kurir can mean:

• a courier
• the courier

The exact meaning depends on context.

In many teaching translations, it may be shown as the courier, but Serbian itself does not mark that with an article.

This is an important difference:

• postao umoran = became tired / got tired
• bio umoran = was tired

So:

• Kurir je postao umoran. focuses on the change of state
• Kurir je bio umoran. simply describes the state

Compare:

• Posle dugog puta, kurir je postao umoran.
  After the long trip, the courier became tired.

• Juče je kurir bio umoran.
  Yesterday the courier was tired.

So postati means a transition into a condition, not just existing in that condition.

Yes. In natural English, postao umoran can often be translated as:

• became tired
• got tired

Both are fine, depending on style and context.

So:

• Kurir je postao umoran. can naturally be:
• The courier became tired.
• The courier got tired.

English often prefers got tired in everyday speech, while became tired sounds a bit more formal or neutral.

This is a question of aspect, which is very important in Serbian.

• postati = perfective
• postajati = imperfective

In simple terms:

• postati focuses on the completed change: to become
• postajati focuses on the process or repeated becoming: to be becoming, to become gradually/repeatedly

In your sentence:

• Kurir je postao umoran.

the speaker presents the change as a completed event, so postati is the right verb.

A sentence with postajati would suggest a process, for example:

• Kurir je postajao sve umorniji.
  The courier was becoming more and more tired.

So postao is the normal choice for a single completed shift into tiredness.

Yes, Serbian word order is more flexible than English, but not all versions sound equally natural.

The neutral word order here is:

• Kurir je postao umoran.

You might also hear:

• Kurir je umoran postao. — possible in poetry or marked style, but not neutral
• Umoran je postao kurir. — strongly marked, unusual, and changes the focus

For normal everyday use, Kurir je postao umoran is the best choice.

What usually stays consistent is that the clitic je wants to be in second position.

Not exactly. Kurir is grammatically masculine because of its noun class, and in this sentence it also refers to a male or grammatically masculine courier.

In Serbian, nouns have grammatical gender, and adjectives and past participles agree with that gender.

If you specifically wanted to refer to a female courier, you would normally use a feminine noun such as:

• kurirka

Then the sentence would become:

• Kurirka je postala umorna.

So the masculine forms here are not random; they are required by the noun used.

This is something learners get used to over time.

The infinitive is:

• postati

The past participle is built from the verb stem and can look quite different from the dictionary form. For masculine singular, many verbs form the participle in -o:

• doći → došao
• naći → našao
• postati → postao

So postao is simply the masculine singular past form connected to postati.

It may look slightly unusual at first because of the vowel sequence -ao, but this is a very common Serbian pattern.

Yes, it is grammatical and natural.

That said, in everyday speech, Serbian speakers might also say things like:

• Kurir se umorio. = The courier got tired.
• Kurir je bio umoran. = The courier was tired.

The version with postao umoran is perfectly correct and emphasizes the idea of becoming tired. It sounds a bit more explicit about the change of state than se umorio, but it is still normal Serbian.