In the first scene, we learn that the sentinels have seen a ghost, and that the ghost they've seen looks just like the dead king. When the ghost appears in Horatio's presence, he immediately recognizes him..... it is the king, and he looks exactly as Horatio remembers him. After the ghost leaves, Horatio notes that Hamlet should be there...... that the ghost would speak to him.
We also learn that the former king was a great rival of Fortinbras, king of Norway. Hamlet, the king's son, did battle with Fortinbras' army and was victorious.... taking Norway from the king. Thus, in this section, we also learn that Fortinbras' son is amassing an army to take back what his father has lost.
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