1. Some prepositions, (short words like 'to' 'on' 'about') can change in Welsh. We've already seen this happening with 'I've got / he's got', etc.

Mae gen i gath
Mae gen ti gath
Mae gynno fo gath
Mae gynni hi gath
Mae gynnon ni gath
Mae gynnoch chi gath
Mae gynnyn nhw gath

But when you use a name, you go back to the original word 'gan' , e,g.
Mae gan Geraint gath

In a similar way

to Geraint = i Geraint
to him = iddo fo
to Siân = i Siân
to her = iddi hi

You don't need to have all these to use at this level, but here's a list of the three most common (i = to; ar = on; am = about). This is for reference only, but when looking at the endings you see a pattern emerge. Not all prepositions change, e.g. with = gyda (South Wales) / efo (North Wales) doesn't change.

i fi
i ti
iddo fo
iddi hi
i ni
i chi
iddyn nhw

arna i
arnat ti
arno fo
arni hi
arnon ni
arnoch chi
arnyn nhw

amdana i
amdanat ti
amdano fo
amdani hi
amdanon ni
amdanoch chi
amdanyn nhw

2. All Welsh nouns are either masculine or feminine but there's no easy way of knowning without looking in a Welsh dictionary. With body parts, if you have two of them, they're generally feminine! e.g. braich (arm), coes (leg), clust (ear). On the other hand trwyn (nose) or pen (head) are masculine.... However, this rule doesn't always work!