Places of work
Colin's broadcasting a radio feature on the listeners' place of work. Here's what the callers had to say...

I work in a shop	Dw i'n gweithio mewn siop
I work in a club	Dw i'n gweithio mewn clwb
I work in a hospital	Dw i'n gweithio mewn ysbyty
I work in a office	Dw i'n gweithio mewn swyddfa
I work in a garage	Dw i'n gweithio mewn garej
I work in a school	Dw i'n gweithio mewn ysgol
I work in TV	Dw i'n gweithio ar y teledu
I work in radio	Dw i'n gweithio ar y radio
I work on a farm	Dw i'n gweithio ar fferm
I work on a boat	Dw i'n gweithio ar long
I work on an oil rig	Dw i'n gweithio ar rig olew

There are two words for in: mewn (in a), and yn (in the).
e.g.
mewn banc : in a bank
yn y banc : in the bank