A stone that would have no liberties (or fill the last liberty of your group) must not be placed.
Unless the very same move captures something. In that case it is allowed (because the capture gives you at least one liberty).
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And because there can never be a stone without a liberty it is possible to create an "immortal" group. But don't worry if you can't imagine how right now, it is a topic for later :).
Actually, let me get back to this one later
EZ, I got this, what's next?