"I've gone to the park yesterday."
Translation:Dw i wedi mynd i'r parc ddoe.
It's a very awkward way of saying something in English too. I would never, nor do I know anybody who would say 'I've gone to the park yesterday'. I agree, it seems ungrammatical. As a Welsh speaker, I can tell you that nobody would ever say 'dw i wedi mynd i'r parc ddoe'. It sounds ridiculous. One would say 'es i i'r parc ddoe' (I went to the park yesterday).