It should be 'Y cefn gwlad' and with the verb 'Mae'r cefn gwlad' = the countryside is

The second part is acting as an adjective describing the subject (the countryside) as 'a beautiful place' so we need the link 'yn'

'in a place, in some place, in place' (non-specific) - mewn lle

• Dw i'n moyn cael fy nghladdu mewn lle llonydd - I want to be buried in a quiet/peaceful place (mewn => in any quiet place)

• Dw i'n moyn cael fy nghladdu yn y fynwent lan y rhiw - I want to be buried in the cemetery up the hill (yn => in a specific place)

• yn eich lle chi... - in your place..., in your shoes..., in your stead...

• mewn unrhyw wlad/dref/le - in any country/town/place
• yng Nhgymru, yn yr Eidal, ym Mangor - in Wales, in Italy, in Bangor
• diddordeb mewn darllen - an interest in reading (non-specific)
• diddordeb yn llên Gymreig - an interest in Welsh literature (specific)