Now that Duolingo has added a male voice for the Portuguese lessons, I'm having a rough go trying to hear the "o" in this sentence. I'm not sure if this new male voice's diction or accent is indicative of a particular area of Brazil or Portugal, but does anyone have any tips for listening to and being able to differentiate between "Eu o vi correndo" and "Eu vi correndo"? Or is the combination of the past tense verb "vi" and the gerund verb "correndo" combined with the assumed context of talking about someone previously enough to determine what is being said? Can you even use the conjugated gerund verb in that way with no subject? I can't for the life of me hear the "o" when the male voice says it, but I can hear it perfectly when the female voice says it.
Someone from Portugal correct me if I'm wrong, but in sentences with a progressive tense like this, the form "a + infinitive verb" is preferred, instead of using the gerund. So: "eu o vi a correr".
"Eu o vi correr" sounds more like "I saw him run". I'm not sure if it's grammatically correct (I think the correct form with the infinitive is "eu vi ele correr"), but you can use it for sure, it doesn't sound strange to a native speaker, just a bit formal.