A simple rule is: it is unacceptable to correct or give instruction to your partner while dancing, unless they ask for your help. This would be a perfect example of humiliating your partner, not exactly a way to encourage people to dance with you.
Do not corner a good dancer and ask for help: they will spend two minutes with you and avoid you forever. If you are asked for a dance you do not know, you can say that you would like to dance but do not know how. This gives the asker a cue to offer help, but at the same time a chance to escape, and hopefully ask for a different dance later.
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