The possessive pronoun follows the object.
In English the third person singular pronoun is he, she or it.
When a noun comes first, it is followed by a relative pronoun, thus, Dafydd a brynodd lyfr yno, which really means " (it is) David who bought a book there," and is never used in any other sense in the spoken language, though in literary Welsh it is used rhetorically for the simple statement which is properly expressed by putting the verb first.
A relative pronoun immediately precedes its verb and can only be separated from it by an infixed pronoun, thus Dafydd a'i prynodd, " (it is) David who bought it," yno y'm gweli, " (it is) there that thou wilt see me."
As Mommsen remarks, the clauses of the sentences are often arranged on the thread of the relative pronoun like thrushes on a string.