How could Luna help Princess Celestia defeat King Somber 1,000 years hace if Luna was in the moon 1,000 years ago??

It happened in the same year, but not exactly the same time. It obviously happened before Luna became Nightmare Moon.

mabey when they defeated king sombra, some of his powers accidentaly ended up in Luna and thats what made her jealous for her sister a few days later.

dont make any cence

Actually, Celestia had dicho "about 1000 years ago", not "exactly 1000 years ago". She wouldn't say "1001 years ago" for sure.

Besides, they might occur in the same year, before she was Nightmare Moon, as most of the fans dicho before me. And Sombra's evil passing to Luna makes sense too.

i do not know