Okay, so prepositional declensions is like how the preposition changed as to which pronoun is next to it. It doesn't happen with every preposition (e.g wrth), but it does in some of the more common ones.
So for example, the preposition 'i'. This means to, most of the time.
However, to say 'to me', 'i i' doesn't work. Therefore it changes to 'i fi'. Then, 'i ti' is fine, but then when you get to 'fe/hi', i becomes 'Iddo'. So 'iddo fe, iddo hi'. Then 'i ni/chi' are both fine, but when you get to 'nhw', it becomes 'iddon nhw'.
This happens with 'o' and with 'am' and with others, so learn these.
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'Sydd' is used as a relative pronoun. It is used to mean 'that/who are doing/have done'. So to say 'This is the boy who is winning', you'd say:
"Dyma'r bachgen sy'n ennill" (sydd yn contracts to 'sy'n')
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When I say relative clauses, I mean sentences like 'I think that he is a boy'
Dw i'n credu ei fod e'n bachgen.
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And mutations, so like remembering to soften the next word with feminine nouns and adjectives (so cath is feminine, so it becomes 'y gath'), with some of the numbers, and some of the possessive adjectives (so work is 'gwaith', and 'your work/job' is 'dy waith di'). And then the rules of aspirates and nasals.
HOWEVER
It is much better that you produce a piece of work with grammar which you are more confident about rather than make big mistakes on grammar you don't know, so use what I've written with a large pinch of salt. I'm just trying to jog your memory if you have learnt about these aspects.
It's MUCH better to be accurate than to be inaccurate, so if you're not sure about what I've written, don't use it.