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Because 'gen' is the conjugated form of 'gan' for the first person singular (i.e., yourself). Saying 'mae gen i wallt cyrliog' says 'I have curly hair'. For she, use 'mae gynni hi', 'mae ganddi hi' or 'mae...gyda hi'.

Yes, but without the mutation - see the examples in the course notes.

• Mae gynni hi wallt cyrliog
• Mae gwallt cyrliog gynni hi

Yes, although then it would be 'gwallt'. The mutation comes after the 'gynni hi'

In South Wales the 'have' bit tends to come after:-

Mae gwallt cyrliog gyda hi.

In North Wales the 'have' bit tends to come before:-

Mae gynni hi wallt cyrliog.

All variations are acceptable

After:- Mae gwallt cyrliog (gyda/gynni/ganddi) hi

Before:- Mae (gyda/gynni/ganddi) hi wallt cyrliog.

The South Wales version of possession is easiest to learn since 'gyda' doesn't change with the pronoun.