Почтовый оператор работает на таможенном посту сегодня.

Because it is the subject of the sentence: the person who is doing the action.

Both words are in the nominative singular masculine:

• оператор = nominative singular masculine noun
• почтовый = adjective agreeing with оператор in gender, number, and case

So почтовый оператор means the postal operator / a postal operator as the subject.

In this sentence, оператор most likely means a worker, clerk, or staff member rather than an operator in the technical English sense.

In Russian, оператор is used in many job contexts. Here, with почтовый, it means someone working in postal services, so a natural English translation might be:

• postal worker
• postal clerk
• postal operator

The exact English wording depends on context.

Работает is the 3rd person singular present tense form of работать.

Why this form?

• 3rd person because the subject is he/she/it in English terms: the postal operator
• singular because there is one operator
• present tense because the sentence talks about the present time

So:

• я работаю = I work
• ты работаешь = you work
• он/она работает = he/she works

It can mean either, depending on context.

Russian present-tense imperfective verbs often cover both:

• works (habitual/general)
• is working (right now / temporarily)

In this sentence, сегодня strongly suggests a temporary situation, so in natural English this is often best translated as:

The postal operator is working at the customs post today.

Without сегодня, it could sound more like a general statement: The postal operator works at the customs post.

Russian often uses на with certain workplaces, posts, stations, and duty locations where English would usually say at.

So:

• на посту = at the post
• на станции = at the station
• на работе = at work

Here, на таможенном посту is the normal way to say at the customs post / checkpoint.

So even though на often means on, here it is better understood as at.

This is a very useful detail. After на or в, some masculine nouns can use a special location form often called the second prepositional or locative.

So with пост:

• regular prepositional form: о посте = about the post
• location form: на посту = at the post

That is why the sentence has: на таможенном посту

This is a normal and idiomatic pattern in Russian.

Because it has to agree with посту.

The noun посту is:

• masculine
• singular
• in a location form after на

The adjective therefore appears in the matching prepositional-type form:

• таможенный пост
• на таможенном посту

So the ending changes because the noun is not in the basic dictionary form anymore.

It means a customs post, customs checkpoint, or sometimes customs office/post, depending on context.

Таможенный comes from таможня, meaning customs.

So this phrase refers to a place where customs control happens, for example:

• checking parcels
• inspecting goods
• processing items crossing a border

Russian word order is more flexible than English word order.

Putting сегодня at the end is completely normal. The sentence could also be rearranged, for example:

• Сегодня почтовый оператор работает на таможенном посту.
• Почтовый оператор сегодня работает на таможенном посту.

These versions all mean roughly the same thing, but the emphasis shifts slightly:

• sentence-final сегодня can sound like the time information is being highlighted or added last
• sentence-initial сегодня puts today more immediately in focus

Russian has no articles.

So почтовый оператор can mean:

• the postal operator
• a postal operator

The listener understands which one is meant from context.

That is very common in Russian. Definiteness is usually not marked directly the way it is in English.

A stressed version is:

Почто́вый опера́тор рабо́тает на тамо́женном посту́ сего́дня.

Main stresses:

• почто́вый
• опера́тор
• рабо́тает
• тамо́женном
• посту́
• сего́дня

If you want to sound natural, it helps to learn the stress together with each new word, since Russian stress is not always predictable.