Inside, speak with Groose and the old woman, then head up to the Gate of Time. Hit it with a Skyward Strike to successfully open a portal to a time millennia ago.
After a quick converse, you’ll rush to the past.
Temple of Hylia
Once you arrive, finally, speak with Impa and you’ll be told to meet up with Zelda in the next room. Go up the nearby staircase to find Zelda, then to watch a very lengthy and plot-revealing scene.
Anyhow, your Master Sword becomes as powered up as it possibly can be. Go to Impa and speak with her again, then go back … to the future!
Sealed Temple (Again)
Back in the present, future … whatever, speak with the old woman and Groose again, then leave the Sealed Temple and find a Bird Statue you can use to go to the sky, then fly northwest to Skyloft.