5. No
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Dogs may not understand why something is off-limits, but they can certainly learn that “no” means stop doing that right now. This word is useful for setting boundaries and discouraging bad behavior, but it needs to be used consistently for it to be effective.
To make “no” more effective, pair it with redirection—immediately guiding your dog toward a desired behavior. Using a firm but calm tone ensures they understand without becoming fearful or anxious.