Hi Kevin
the problem here is that the English “to” has different meanings. If it marks only the infinitiv of a verb, like : I want to do , I like to eat etc. you don’t need the “i” in Welsh.
But somettimes “to” shows you something like a direction, then you use the Welsh “i”
It is normally the word before the “i” that causes the use of it, so it’s maybe to easier to learn:
I’m going to do something Dw i’n mynd i wneud
How to do it Sut i wneud o
I want to do something. Dw i isho gwneud
I’m ready to sleep Dw i’n barod i gysgu.
And as always, don’t worry too much about it. You’ll be understood anyway.