sy is a special third-person form of the present tense of bod (being, to be). bydd/fydd is the future tense, which does not have a special form to match sy:
- Siân sy yno - It is Siân who is there
- Siân fydd yno - It is Siân who will be there
- Siân oedd yno - It is Siân who was there/used to be there
- Siân fasai yno - It is Siân who would be there
Diolch yn fawr, dw i'n meddwl mod i'n deall. As soon as I posted this question, I started to think it may have something to do with the tense.
But is sy really just a third-person form? I recall seeing sentences using sy with first- and second-person subjects here on Duolingo, along the lines of Ti sy 'na? or Fi sy biau hwnna.
Yes, sy only exists for the third person singular. With rather peculiar English:
- Fi sy'n mynd i Aber, nid ti - Me it is who is going to Aber, not you
where the '...who is...' indicates the third person nature of sy - anybody or anything could replace Fi here (another you, Sioned, Father Christmas, they, he, a cat, the boxes, ...) and sy would still be used.