In Welsh, the preposition must have a suffix (an ending) to match the pronoun that follows it, e.g.

wrth (preposition) + hi (pronoun) = wrthi
gan (preposition) + hi (pronoun) = ganddi

There is no suffix if the preposition is followed by a noun, e.g. chwilio am y dillad, mynd i’r brifysgol.

The suffixes for the prepositions can be found by clicking here.