"Cas pensiliau mawr"
Translation:A big pencil case
That's close to the above English phrase, although a closer translation for the two would be 'a big case of pencils' .
Although we say 'pencil case' in English, the Welsh expression is 'cas pensiliau' (lit:- a case of pencils). Since we don't have an adjective form of pencil. If we did it would be something like 'cas pensilaidd'. But that would really mean 'a case in the form of a pencil'.
Or maybe the English is a contraction of 'pencils' case'.