Wieczorem nie było nikogo na balkonie, bo zaczęło padać.

Wieczorem is a very common Polish way to say in the evening. It is an old instrumental-case form of wieczór that functions adverbially, so it does not need a preposition.

So:

• wieczór = evening
• wieczorem = in the evening / during the evening

This is similar to other time expressions in Polish, for example:

• rano = in the morning
• nocą = at night / during the night

So in this sentence, Wieczorem simply sets the time: In the evening.

Because this is an impersonal/existential construction: nie było nikogo = there was nobody.

In Polish, when you say that something or someone was / wasn't present, and you are not making a specific noun the grammatical subject, Polish often uses było, the neuter singular past form of być.

So:

• Był mężczyzna = There was a man
• Była kobieta = There was a woman
• Nie było nikogo = There was nobody

Here, the sentence is not built around a subject like nikt. Instead, it uses an existential structure, so było is the natural form.

Because after nie było, Polish normally uses the genitive case.

• nikt = nobody, no one
• nikogo = genitive/accusative form of nikt

In negated existential expressions such as:

• nie ma nikogo = there is nobody
• nie było nikogo = there was nobody

the word for nobody appears as nikogo.

So nie było nikogo is the standard pattern.

Yes, you could, and it would also mean Nobody was on the balcony.

The difference is mainly in structure and nuance:

• Nie było nikogo na balkonie = There was nobody on the balcony

  • more existential
  • focuses on the absence of any person there

• Nikt nie był na balkonie = Nobody was on the balcony

  • more personal/subject-based
  • focuses on nobody as the subject

Both are correct. The version in your sentence is very natural and common.

Because na can take different cases depending on whether you mean location or movement.

Here, na balkonie means on the balcony as a location, so it uses the locative case.

• na balkonie = on the balcony (location)
• na balkon = onto the balcony (movement/direction)

Compare:

• Nikogo nie było na balkonie. = Nobody was on the balcony.
• Wyszedł na balkon. = He went out onto the balcony.

Because Polish does not need a dummy subject like English there.

English says:

• There was nobody on the balcony

But Polish simply says:

• Nie było nikogo na balkonie

The idea of there was / there were is expressed just by the verb and the sentence structure. Polish often leaves out elements that English requires grammatically.

Zaczęło padać means it started to rain or it began raining.

Here:

• zaczęło = started / began
• padać = to fall; in weather contexts, to rain, to snow, etc.

So padać is the normal verb used for rain in Polish:

• Pada. = It’s raining.
• Zaczęło padać. = It started to rain.

This is the natural Polish way to express the idea.

For weather expressions and other impersonal statements, Polish often uses a neuter singular verb form.

English uses a dummy subject:

• It started to rain

Polish does not need that it, but the verb still appears in a default impersonal form:

• Zaczęło padać

So the -ło ending does not refer to a real neuter noun here. It is just the normal form used in this kind of impersonal sentence.

Bo means because.

So the structure is:

• Wieczorem nie było nikogo na balkonie = In the evening there was nobody on the balcony
• bo zaczęło padać = because it started to rain

Bo is very common in everyday spoken and written Polish. It is slightly more conversational than ponieważ, which also means because.

Because in Polish, a clause introduced by bo is normally separated by a comma.

So:

• ..., bo zaczęło padać.

This is standard Polish punctuation. It works like English punctuation before a because clause in many similar sentences.

Polish word order is fairly flexible, but the version you have is very natural.

Wieczorem is placed first to set the scene: In the evening...

Then comes the main statement:

• nie było nikogo na balkonie

And then the reason:

• bo zaczęło padać

Other word orders are possible, but they would change emphasis. For example:

• Nie było nikogo na balkonie wieczorem, bo zaczęło padać.

This is still understandable, but the original sounds smoother and more natural as a neutral sentence.