"I said that you would be right."
Translation:Dwedais i y baset ti'n iawn.
y and mai/taw are not interchangeable. mai/taw is used when the sub-clause is an emphatic one, with the verb following the element being emphasised in the sub-clause:
- Dwedais i (y) baset ti'n iawn - I said that you would be right. (no emphasis)
- Dwedais i mai ti (a) fasai'n iawn - 'I said that YOU would be right' or 'I said that it would be you who was right.' (The relative pronoun a here is usually dropped, but it is that that requires the verb following to be in the third person)
So, the third-person verb in the sub-clause is sy in EllisV's example in the present tense, and fyddai in my example in the conditional tense. Compare:
- Rwyt ti'n gywir - You are correct. (not emphatic, second person verb)
- Ti sy'n gywir - You are correct (or, in awkward English, 'You it is who is correct') (Emphatic, with third person relative verb sy.)
- Baset ti'n gywir - You would be correct
- Ti (a) fasai'n gywir - You would be correct (or 'You it is who would be correct') (emphatic, with third person verb following ti, the emphasised element, and with a soft mutation required after this use of a as a relative pronoun.)
(Note that sy/sydd does not require the relative pronoun a - its function is included in the sy/sydd form.)
Whereas sy is a fairly simple pattern, the use of other verbs and tenses in an emphatic sub-clause is a more complex point of Welsh grammar than is usually covered at this level!