Mewn - In (a)
'Mewn' means 'in' but is used exclusively when the noun which follows it is non-specific. Consequently we use it whenever we want to say 'in a'. Specific nouns require the use of yn. e.g.
Mewn does not have inflected forms and it does not cause mutation.
Useful phrases: mewn gwirionedd - in fact , actually | dewch i mewn - come in