I wonder what is this thing that takes a special skill to do with a knife
Shaving in the wilderness. It's a lost art.
Eating soup? A guy in another exercise was trying it, but that must have been a different guy. THIS person clearly knows what a challenge that can be!
Sharpening a pencil?
Gutting an animal fish when in the wild.
"not able to do it with a knife" is wrong?
That is not even a complete sentence, you are missing "I am " or something similar. You should certainly also include a translation for umět.
How does the 'neumím' cover both 'i don't know' and even 'how'?
So 'neumím' translates to: 'I do not know how'. Could 'neumím' in another context translate to 'don't know when' or '..what' etc.?
Well, that is just the difference between the too languages. English requires the "how" here, Czech does not.
Would "Ale já to..." instead of "Já to ale..." be correct?
Yes, that's definitely absolutely okay. This exercise is showing you (among other things) that "ale" does not have to stand at the beginning.