once you start something, you must finish it
Man muss eine einmal begonnene Handlung oder Entscheidung konsequent zu Ende führen.
The expression means that if someone has started something or made a decision, they must follow through to the end.
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Wenn du das Projekt startest, musst du weitermachen; wer A sagt, muss auch B sagen.— If you start the project, you have to keep going; once you start, you must finish it.