Why does 'yn' come before "hoffi" and not 'bacwn'?
It's a particle that links a predicate (such as the verb hoffi) to a form of bod ("to be") such as dw. So it comes before the predicate.
It can also come before other kinds of predicates besides verbs, especially adjectives: Mae Siân yn hapus "Siân is happy".
...Said no one ever
Although vegetarians (llysieuwyr) may have this opinion.