You say _a_ lie and _the_ truth is pretty easy, but you're coming from the position that there is always one truth and there can be many lies.
Our culture expresses truth and falsehood as a dichotomy in which there is only one truth and many falsehoods. This is not real, though—it's just something that exists in your mind. You could choose to look at it another way, and if that was how you were taught and it was idiomatic in your language, then you would also say it was easy.
Here your intuition is not shaping your understanding of truth and falsehood—culture and language have shaped your understanding.
So whether you're learning to express truth and falsehood idiomatically in a language, or whether you're learning how to use articles or not use articles, it is only easy or hard depending on how your culture has shaped your perception of things.
Our culture expresses truth and falsehood as a dichotomy in which there is only one truth and many falsehoods. This is not real, though—it's just something that exists in your mind. You could choose to look at it another way, and if that was how you were taught and it was idiomatic in your language, then you would also say it was easy.
Here your intuition is not shaping your understanding of truth and falsehood—culture and language have shaped your understanding.
So whether you're learning to express truth and falsehood idiomatically in a language, or whether you're learning how to use articles or not use articles, it is only easy or hard depending on how your culture has shaped your perception of things.