"Is it useful?"
Translation:Czy to jest przydatne?
If there was a conversation about a computer, for example, the context would tell us that "komputer" is masculine, so when I ask about it in a separate sentence, like this above, would "Czy to jest przydatnY" work, or could I say "Czy TEN jest przydatnY", or still only "Czy to jest przydatne" wold work?
"ten" in "Czy ten jest przydatny?" would not exactly mean "this" (note that "Is [this/that/it] useful?" mean basically the same) but "this one".
So I guess you already got an answer that Computer A is not useful, you point at Computer B and ask "Is this one useful?". So it is in fact a different sentence.