Wnei comes from gwneud i.e to do/make whereas fyddi di is a form of bod. So "Wnei di siarad Cymraeg?"= Will you speak welsh? , "Fyddi di'n siarad Cymraeg"=Will you be speaking welsh?
Thanks, so practically is there any difference in meaning between the two? Can i pick whichever one i like most and use that? Or are there times when you have to use one and not the other?
If you want to ask a favour, you use the wnei di form. So Wnei di yrru'r car? is like saying "Will you / Would you mind / Could you drive the car?". Fyddi di'n gyrru'r car is just an enquiry about the future, "Will you be driving the car?". You could think of the Wnei di... form as a polite command almost.
Careful with the answers too:
Wnei di yrru'r car? / Gyrra'r car! answer: Gwna(f) (i) / Na wna(f) (i)
Fyddi di'n gyrru'r car? answer: Bydda(f) / Na fydda(f)
I feel like "Wnei di" is almost like how in English you would say "would you go to the shop for me" whereas "Fyddi di" is more "Will you go to the shop for me. So I think they're interchangeable but don't quote me on it.
They're not always interchangeable.
- "I will be" = bod
Bydda i 'na "I'll be there"
Bydd hi'n oer "It'll be cold"
- Asking a favour or making an offer = gwneud
Wnei di ddod hefyd? "Will you (please) come too?"
Gwna i heplu "I'll help" (as in, you're offering to help)
The bod forms just ask for information.
Fyddi di'n dod hefyd? "Will you be coming too?"
Bydda i'n helpu "I'll be helping"
- Determination, immediacy = gwneud
Gwna i weithio arno fe heno "I'll work on it tonight"
Bydda i'n gweithio arno fe heno "I'll work/be working on it tonight"
The first sounds a bit more determined.
Gwna i dalu "I'll pay"
Bydda i'n talu "I'll pay/be paying"
The first has more immediacy, more reference to the near future.
- Otherwise they seem to be interchangeable.
Gwnaiff / Gwneith hi fwrw fory / Bydd hi'n bwrw fory "It'll rain tomorrow"
I'm thinking the distinction is more: 'wnei di?' is as you say 'will you' and 'fyddi di' a more general 'are you going to...'. at least this is how I'm choosing to distinguish these...