"Some spiders have eaten him."
Translation:Sežrali ho nějací pavouci.
That is quite subtle.
Nějací - some unidentified or previously unknown.
Někteří - some of the spiders (which we may know already).
But it is better to Czech a good dictionary with explanations and check some actual usage examples.
I think I can imagine a context for někteří so I will add it. Although it may be a bit gross. Like if you served pieces of someone to a group of spiders end check if some will eat him...
But probably only in "Někteří pavouci ho sežrali.", not in your word order.