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However, I found no better way with irregular plurals than to just get them to learn them, one to three new ones a week (depending on their level).
Obviously, simple pluralisation doesn't really need to be taught - "add an s" doesn't take much learning! "Y" becomes "ies" also doesn't take too long to sink in.
Maybe someone else found a better way but I just told my students that they would simply have to learn that it's "mice" not "mouses" - we used example sentences or experiences from their own lives that they could tie the words in to.