"A girl was injured."
Translation:Gaeth hogan ei hanafu.
What is wrong with the combination Gaeth merch, as opposed to the correction Gaseth merch? Is this me mixing between dialects?
I'm not sure where you've found Gaseth, but I think that might be the problem since I've never come across it before.
There are lots of words in Welsh for girl. I think on this course there are at least two - hogan and merch. Either should be accepted wherever 'girl' is required.
For caeth you should be able to use any of caeth, gaeth, fe/mi gaeth, cafodd, fe/mi gafodd.