(S) I can't untie that knot with one hand.
Even though you have probably never seen this sentence, you can easily see things like the following:
The sentence is about the abilities of whoever spoke or wrote it. (Call this person the speaker.)
It's also about a knot, maybe one that the speaker is pointing at.
The sentence denies that the speaker has a certain ability. (This is the contribution of the word ‘can't'.)
Untying is a way of making something not tied.
The sentence doesn't mean that the knot has one hand; it has to do with how many hands are used to do the untying.