Rano okazało się, że drugi pojemnik z resztkami został w pracy.

Rano means in the morning.

In Polish, rano is an adverb, so it works on its own without a preposition. That is why you say:

• Rano okazało się... = In the morning, it turned out...

Compare:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night

So this is just a normal time expression, not a noun phrase like w poranek.

Okazało się means it turned out or it became clear.

This is a very common Polish expression:

• okazać się = to turn out / to prove to be

Examples:

• Okazało się, że miał rację. = It turned out that he was right.
• Okazało się, że sklep był zamknięty. = It turned out that the shop was closed.

In your sentence:

• Rano okazało się, że... = In the morning, it turned out that...

Because here the phrase is being used impersonally, like English it turned out.

Polish often uses neuter singular past tense in this kind of structure:

• okazało się, że...

The -ło ending is neuter singular past. There is no real concrete subject here; it is similar to the dummy it in English it turned out that...

If there were a specific masculine subject, you could get:

• Plan okazał się dobry. = The plan turned out to be good.

But in your sentence, the structure is:

• okazało się, że... = it turned out that...

In this expression, się is part of the verb phrase okazać się.

You should learn it as a set expression:

• okazać się = to turn out / prove to be

Even though się often looks like oneself, in many Polish verbs it does not translate literally. It just belongs to the verb.

So here:

• okazało się does not mean something like showed itself
• it simply means turned out

Because że introduces a subordinate clause, and in Polish that clause is normally separated by a comma.

So:

• okazało się, że drugi pojemnik...

This is standard Polish punctuation.

Compare:

• Wiem, że przyjdzie. = I know that he will come.
• Myślę, że to dobry pomysł. = I think that it’s a good idea.

Whenever że means that and introduces a clause, a comma is usually needed.

Usually drugi means second.

So drugi pojemnik most naturally means:

• the second container
• or the other container, if the context is about two containers

In real life, if someone had brought two food containers to work, drugi pojemnik could naturally mean the other one.

Grammar:

• drugi = adjective
• pojemnik = noun
• both are masculine singular nominative here

So they agree:

• drugi pojemnik

Resztkami is in the instrumental case because of the preposition z meaning with.

Here:

• pojemnik z resztkami = a container with leftovers

Case pattern:

• z
  • instrumental = with

Examples:

• kawa z mlekiem = coffee with milk
• kanapka z serem = a sandwich with cheese
• pojemnik z resztkami = a container with leftovers

About the form itself:

• base form: resztki = leftovers
• instrumental plural: resztkami

Because Polish normally uses resztki as a plural noun when talking about leftovers.

So:

• resztki = leftovers
• z resztkami = with leftovers

This is similar to English, where leftovers is also usually plural.

Here został means was left or ended up remaining.

The verb zostać often means to stay / remain / be left depending on context.

So:

• drugi pojemnik został w pracy = the second container was left at work

It does not necessarily mean someone intentionally left it there; it often implies that it simply remained there.

Why został?

Because pojemnik is masculine singular, so the past tense is masculine singular too:

• został = masculine singular past
• compare:
  • torba została = feminine singular
  • jedzenie zostało = neuter singular

Not with the same meaning.

• był w pracy means was at work
• został w pracy here means was left at work / remained at work

So these are different ideas:

• Pojemnik był w pracy. = The container was at work.
  (just location)
• Pojemnik został w pracy. = The container got left at work / remained at work.
  (suggests it was not brought back)

So został is the better choice for the meaning of something accidentally being left behind.

Pracy is in the locative case because it follows w meaning in / at when talking about location.

So:

• w pracy = at work
• w
  • locative = location

Base form:

• praca = work

Locative form:

• pracy

Compare:

• w domu = at home
• w szkole = at school
• w pracy = at work

Yes, Polish word order is flexible, but the given version sounds natural and neutral.

Current order:

• Rano okazało się, że drugi pojemnik z resztkami został w pracy.

This puts the time first, then the phrase it turned out, then the clause explaining what turned out.

You could also say:

• Okazało się rano, że drugi pojemnik z resztkami został w pracy.

This is possible, but a bit less natural in many contexts. Starting with Rano sounds smooth and common.

Polish often moves elements around for emphasis, but the original order is a very normal default.

No, the sentence does not say who left it.

That is very normal in Polish. The sentence focuses on the fact that the container was left at work, not on the person responsible.

English also often does this:

• It turned out that the second container was left at work.

If you wanted to say who left it, you would need a different structure, for example:

• Rano okazało się, że zostawiłem drugi pojemnik z resztkami w pracy.
  = In the morning, it turned out that I had left the second container with leftovers at work.

Yes, pojemnik is a very natural general word for container, especially for a food container, storage container, or plastic box.

Depending on context, Polish speakers might also say:

• pojemnik
• pudełko
• lunchbox (informally, especially in modern speech)

But pojemnik z resztkami is perfectly natural and clear.