"He played with the computer all afternoon."
Translation:Celé odpoledne si hrál s počítačem.
so the computer is viewed as some kind of playing partner and not as an instrument played with??????
Actually, I'd say hrát (hry) na počítači 'to play (games) on the computer' or hrát počítačové hry 'to play computer/video games' is more common. I suppose that is what you are asking about.
Hrát si s počítačem sounds to me as if you were 'playing' with the computer physically/tinkering with it.
You mean Hrát si s počítačem. Yes, playing with it like with a toy or doing some upgrades inside...
Here, in the Czech sentence, it is like a toy. Similar to "Celé odpoledne si hrál s míčem." = "He played with the ball all afternoon."
Like a partner would be "Celé odpoledne hrál s počítačem." The English stays the same and can possibly mean both. Or against computer, "hrál proti počítači".
Musical instruments use "hrát na": "Celé odpoledne hrál na piano."
well, my question was more about why there is thar "s(e)". why not a naked instrumental such as in "jezdit autem"?
This simply differs from verb to verb. Some use varios prepositions and some use direct objects. Some change their meaning between these two versions: Hrál tenis, hrál na kytaru, hrál si s hračkou. He played tennis, he played the guitar, he played with a toy.
For playing video games on a computer we usually say: Hrát na počítači."