Complex sentence: Finally, the main protagonist of the story describes how very small changes to the letter caused it to be hidden from the police's eyes, even when they saw it, simply since it didn't look like what they thought it should.
Split into simpler sentences: Finally, the main protagonist of the story describes how very small changes to the letter caused it to be hidden from the police's eyes, even when they saw it. Simply since it didn't look like what they thought it should.