Why is it sometimes “sydd yn” (it always seems to be “syth yn moyn”) but sometimes it is just “sy’n” (it never seems to be sy’n moyn")?
It’s not much different than “isn’t vs is not” in english as far as I know. It can be “sydd yn” for a little emphasis.
In the north we don’t use the “yn” with eisiau/isio (and a few others) so we don’t have that same issue with moyn.
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In the same way as English sometimes has ‘it is’ and sometimes has ‘it’s’… 
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Whoops, I used the negative in english but you get my drift. 
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