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

o + nhw = ohonyn (preposition) nhw (pronoun)

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.