>I can't figure out how someone could easily write something like that musically speaking
But who said it was easy? It took Mark 3 months to write Romeo And Juliet and that's an "easy" 5-chord song compared to 30-chord (!) Love Over Gold. Not only Mark was able to write it, but he also brought it to the musicians and explained how it works, which requires serious chops in understanding music theory (genius musicians can learn all this through practice alone, mere mortals can learn it through books as well).
>Maybe it's the fact that I'm just starting learning music but it seem to me pretty hard songwriting right here
Yes and yes.
>I love the song and the more I learn it the more I love it its kind of crazy
That looks like the definition of the word "classic" to me.
***
This is indeed a very beautiful song, and it's pretty much piano-oriented. The very first chord of the verse, F/A, sounds pretty dull on the guitar and is not common, especially as a first chord, but it sounds amazing on the piano. It sounds at home in this song too, and reminds me of another piano-based title track Mark wrote a decade later — On Every Street, which starts with F/C and sounds even duller on guitar.
When a songwriter writes songs that work the best not on their primary instrument, it's a sign of a high level of their craftsmanship. How do they do it? Just listen to the "Bridge Over Troubled Water" Demo by Paul Simon he recorded on one acoustic guitar. The WHOLE song is there, it's just a little bit different. And in this case, unfortunately, even though Larry Knechtel made an amazing arrangement of this song for the piano, it's such a strong song, no contribution would make anyone its co-writer.