Grammatical difference please between am and mewn. 
Both am and mewn are prepositions, but with different meaning. One meaning of am is for, in sentences such as
Dw iān chwilio am anrheg ā I am looking for a present
and mewn means in or inside, for example
Dw iān gweithio mewn swyddfa ā I work in an office
But especially am can also be used in different contexts, resulting in other meanings, so in order to help you better, it would be best if we knew about the context.
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