Translation:I want a dog; I already have a cat.
It's the same in English really. The sentence can be extremely long, but as long as the clauses are properly set off by punctuation, it's not incorrect.
This sentence in English is correct: I want a dog; I already have a cat.
This is not: I want a dog I already have a cat. (no punctuation or connecting word)
Would this be a run on sentence in Russian? (I assume it would). Я хочу собаку кошка у меня уже есть.
it would be just an incorrectly punctuated sentence. Similar to how "I bought an apple an orange a hat and a cat" is not a run-on sentence.
The thing is, comma is considered a "proper" way to separate clauses. Such commans are mostly required by conventional rules when you connect two sentences, unless someting QUITE tricky is going on, like two clauses having a part in common:
- У нас в городе строятся дороги и проектируются новые районы. = In our city, roads are being built and new districts are being planned for construction.
In this case you cannot even claim these are two independent clauses because "in our city" is a part of both statements.
So the only way to make a run on sentence with improper punctuation is to make a sentence without essential punctuation there. This still leaves place for sentences like "I have to go, fish are animals and since we came, too much time has passed, they read for long". Which is crap writing, not a run on sentence :)
Russian does not use the Oxford comma. The correct punctuation looks as follows:
- Я купил бананы и апельсины.
- Я купил бананы, гранаты и апельсины.
- Нужно купить бананы, яблоки или груши.
- Я купил и бананы, и апельсины.
- Я не купил ни бананы, ни апельсины.
- Я купил бананы и пошёл домой.
- Я купил бананы, и это замечательно.