Certainly, the answer I have found is:
The verb to be (bod in Welsh) has three principal uses:
i. identification (who/what is/are)
ii. existential (there is/are)
iii. descriptive (anything else)
English makes no distinction between i and iii, but does have a special form for ii (there is/there are). However, in Welsh there is a distinction in all three cases.
Futhermore, in the identification sentences, the 3rd person singular distinguishes between the element which is being enquired about (subject, object or plain identification):
Pwy ydy/yw hwnna?
Who is that? - Identification
Pwy sy'n helpu gyda'r llestri heno?
Who is helping with the dishes tonight - Pwy is the subject (person doing the action)
Pwy mae Sioned yn helpu gyda'r llestri heno?
Who is Sioned helping with the dishes tonight - Pwy is the object (the person being helped by Sioned(subject)
I hope this explaination is clear enough for people to understand.
OBOD Member formally known as Panoramix /|\