He gave them much advice about food and water and the journey, but the most important and the last thing he told them was 'Stay on the path!'
HE warns them to never ever leave the path and to never drink or eat from the forest
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The biggest warning was to stay on the path. Once they left the path they were lost, captured by spiders and then by the elves.
He proves himself useful on several occassions. He rescues them from the spiders in Mirkwood. His warning allowed Gandalf to escape and rescue them in the Goblin Cave. He gets them out of the elf dungeon in Mirkwood. And then he gets them into the Mountain.
The dwarves disregard Beorn's warning about the stream and cross it in a boat.
Green Day sings a song called warning
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