Student or Learner
Why ,,shon'' is not spelled like....shon but it's spelled ,,seen'' ???
I think you're talking about the name "Sean" - as in "Sean Bean" (pronounced, confusingly, /ʃɔ:n bi:n/ ).
The reason is that the name was originally borrowed from Gaelic. It means 'John'. So do 'Jean', 'Jan', 'Ian', 'Ivan'....
The fact that English has borrowed from so many other languages makes our spelling very hard to master. The "-ea-" in Sean, bean, measure, pear, and break all make different sounds; the "-ea-" in [a dog's] "lead" and the metal "lead" make different sounds.* You just have to learn these - when you first meet them in writing, there's no way of knowing which pronunciation is right. I'm sorry, but that's the way it is .
PS *These words don't all come from different borrowings; there are other reasons behind these apparent inconsistencies. And there are probably other pronunciations for the spelt letters "-ea-"; these are just ones that came to mind.