I want to know how to say:
You should sit down and please stop moving.
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I just remembered this particular kind of imperative (actually 'mode indicatif') that is sometimes used with children and pets. It conveys a stronger command than just the imperative mood. Point with finger for greater effect.
Tu t'asseois et tu arrêtes de bouger.
This is somewhat analogous to english 'You don't talk back to me' used as imperative.