In English would it also be acceptable to translate as "would you like a ball?" Not a direct translation but surely used interchangeably in English?
I believe that it is different as 'Do you want?' is a statement turned into a question, whereas 'would you like?' is a question in the conditional tense.
"I don't want A toast" isn't correct, so the next question -this one- I answer "do you want ball" without the "a", and the correct answer is with an "a". WHY????
Because you cannot say "a toast" in English. In this case, it has nothing to do with Welsh. "Toast" is not countable, unless you say "a piece of toast."