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I am sitting here editing a text written by a non-native English speaker, who keeps using the word "training" in plural. For instance: "A number of vocational trainings..." Being a non-native English speaker myself, I can't say exactly why that sounds so terrible, but it definately does. I have brought up various alternatives - like training sessions, training classes, etc. - but the writer keeps on using "trainings."

So, what I need are some good pointers as to why this is frowned upon and, ideally, why it is simply wrong.