I don’t understand why the past tense and present tense are combined when saying “You said that you wanted” which would be, if I understand correctly, "Ddudest ti bo’ ti eisiau (NW). Could it also be “Ddudest ti oeddet eisiau”
Not really. You might hear some people say that but it is incorrect - it’s following the English pattern, not the Welsh one. In Welsh the tense is set by the first part (“you said”) and you don’t need to amend the tense of the second part to match like you do in English (“wanted”).
It may help to think of it like this - in Welsh you can hear quotation marks!
Me, today: I want a balloon
You, tomorrow: you said “I want a balloon”
notice the use of quotation marks - they negate the need to change want to wanted, and that’s what we’re actually copying in Welsh.
In English the quotation marks aren’t considered so the tenses have to be made to match to make sense.
Sometimes we just have to resist the temptation to translate word for word and tense for tense!
That was very helpful, thank you. Does that also apply to the future tense as in: “Bydda i ddeud bo fi’n mynd i’r ysgol” meaning 'I will be saying that I will go to school" . Or is it “Bydda i’n ddeud bydda i’n mynd i’r ysgol” or am I just totally screwing it up?
No, it’s different with the future tense.
Going back to our ‘quotation marks’, the bit inside them is in the present tense, so you use the ‘bod’ for ‘that’.
With your future sentence, the bit that would be in ‘quotation marks’ (i.e. the bit after the ‘that’), is in the future tense, so you can’t use ‘bod’ here, you need to use ‘y’ - Bydda i’n deud y bydda i’n mynd i’r ysgol (but because the ‘y’ is such a little sound here it can often get lost in speech and sound like it’s not there at all!)
Very helpful and clearly stated. Thank you very much.