Foreword: p 198, Plain Words, 2014, by Ernest Gowers, revised by Rebecca Gowers, introduced me to the covert danger of 'Cannibalism by prepositions'. Then a Google search directed me to this Wordreference.com post about which I ask now.
✘ 1. The letter is not from whom it claims to be.
✓ 2. The letter is not from whom it claims to be from.
✓ 3. The letter is not from the person from whom it claims to be.
Initially, I couldn't accept that 1 was wrong. I had to work through this step-by-step proof, before I accepted that 1 was wrong, and only 2 and 3 are right. Now, I can prove that 1 is wrong; so I ask NOT about the aforesaid proof.
Instead, my problem is this: Even after seeing and working through the aforesaid proof, 1 STILL looks right to me, when it shouldn't! In other words, could someone please explain how to understand (or intuit) that 1 is wrong, WITHOUT referring to the aforesaid proof?