1. I packed my bag the night before my trip.

2. I packed my bag at the night before my trip.

3. The principal was in the hall looking in at our class.

4. Don't wake the baby.
5. Don't wake up the baby.

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Which ones do we have to use? Are they all grammatical? If there are errors, would you correct them? Thank you.

2 is incorrect. The others are fine.