Do not criticise without specifying what results you want to see. If, say, a colleague sends you an overlong presentation, instead of responding only: "That it is too long", say instead: "You are on the right lines and if you could just shorten it to 10 slides, it'll be perfect."
Imperatives in an e-mail sound very authoritarian. Preference should be given to conditionals. Rather than: "Do it, finish it, arrange it", a better approach is: "Could you do it, finish it, arrange it?" Recipients then justifiably feel that it is up to them how they fulfil the task.
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Article source Harvard Business Review - flagship magazine of Harvard Business School