It’s also a little confusing, because Level 1 of SSIW (northern) uses gofyn wrthot ti etc, but when you get to Level 2, they drop that in favour of gofyn i ti. There’s an old thread on this forum which suggests that gofyn i is more ‘correct’, but gofyn wrth is common in speech: Gofyn i or gofyn wrth or both.
In general, prepositions (such as wrth and i) are a difficulty in every language and you’ve just got to learn them.
For example, the French ‘listen the radio’, in English we ‘listen to the radio’ and in Welsh, we 'listen on the radio… (gwrando ar Radio Cymru).
Welsh just happens to use wrth with dweud, wrth or i with gofyn and nothing with helpu and that’s the way it goes – you do start to get used to which to use after a while, but if you forget, people will still know what you mean.