Danas je stupio na rad.

Yes. Stupiti na rad is a set phrase, especially in formal or administrative Serbian.

• stupiti literally means something like to step, to enter, or to take up
• na rad literally means into work / onto work

But together, stupiti na rad does not usually sound physical in the sense of literally stepping somewhere. It means:

• to start work
• to begin employment
• to take up work duties

So this is best understood as an idiomatic, fairly formal expression rather than translated word-for-word.

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

In Danas je stupio na rad:

• Danas = first position
• je = clitic, so it comes next

This is very normal Serbian word order.

Compare:

• Danas je stupio na rad.
• On je danas stupio na rad.

In the second sentence, On takes first position, so je comes right after it.

So the placement of je is not random; it follows the common clitic-placement rule.

It is the perfect tense, which is the most common past tense in modern Serbian.

It is formed with:

• the auxiliary biti in the present tense
• plus the past active participle

Here:

• je = is/has as the auxiliary
• stupio = past participle

So grammatically, this is the Serbian perfect, but in meaning it usually corresponds to a simple English past:

• He started work today
• He began work today

Serbian uses the perfect very broadly for past events, including completed actions earlier the same day.

Stupio is the masculine singular past active participle of the verb stupiti.

It agrees with the subject in gender and number.

So if the subject changes, the participle changes too:

• On je stupio na rad. = a man / masculine subject
• Ona je stupila na rad. = a woman / feminine subject
• Ono je stupilo na rad. = neuter subject
• Oni su stupili na rad. = masculine or mixed plural
• One su stupile na rad. = feminine plural

So from stupio, you can tell the hidden subject is masculine singular.

Serbian often leaves the subject unstated when it is clear from context.

In this sentence, there is no explicit subject, but grammar gives you clues:

• je tells you it is 3rd person singular
• stupio tells you the subject is masculine singular

So the sentence implies something like:

• he started work today
• or the man/person mentioned earlier started work today

This omission is very common in Serbian and sounds completely natural.

Here rad is in the accusative singular.

The phrase uses na + accusative because it expresses movement or entry into a state/activity: stupiti na rad = to enter/start work.

A useful contrast is:

• na rad = starting / going into work in this formal sense
• na radu = at work, with location/state, so na + locative

For example:

• Stupio je na rad. = He started work.
• On je na radu. = He is at work.

Also, rad is a masculine inanimate noun, and in the singular its accusative form looks the same as the nominative, so you see rad, not a visibly different ending.

It is understandable and correct, but it sounds formal or administrative.

You often see this kind of wording in:

• HR documents
• official announcements
• legal or bureaucratic language
• formal reporting

In everyday speech, people more often say:

• Danas je počeo da radi.
• Danas je krenuo da radi.

So:

• stupio na rad = more official/formal
• počeo da radi = more neutral and conversational

They are close in meaning, but the tone is different.

Stupio na rad:

• more formal
• often implies formally beginning employment or officially taking up work
• common in institutional language

Počeo da radi:

• more general and conversational
• simply means started working
• can refer to a job, a task, a machine, or a process depending on context

So if you are talking casually, počeo da radi is usually the safer choice. If you are reading documents or formal notices, stupio na rad is very natural.

Because this expression is a fixed formal phrase: stupiti na rad.

Although rad and posao can both relate to work, they are not interchangeable in all expressions.

• rad often refers to work as activity, labor, or employment in a formal sense
• posao more often means job, task, or workplace-related work in everyday language

For example:

• ići na posao = to go to work
• početi posao = to begin a task/job
• stupiti na rad = to formally start work/employment

So na posao and na rad are not the same thing here.

Yes, Serbian word order is flexible, but not every version sounds equally natural in every context.

Possible versions include:

• Danas je stupio na rad.
• Stupio je na rad danas.
• Na rad je stupio danas.

They all point to roughly the same basic meaning, but the emphasis changes.

• Danas je stupio na rad emphasizes today
• Na rad je stupio danas puts more focus on the act of taking up work
• Stupio je na rad danas is possible, but may sound less neutral depending on context

The original sentence is a very normal, neutral way to say it, especially in formal style.