Simplify the following text for a Grade 6 native English-speaking person.
Original: Goryeo expanded its borders to present-day Wonsan in the north-east ( 936 ~ 943 ) and the Amnok River ( 993 ) and finally almost the whole of the Korean peninsula ( 1374 ) .
Simplified: Goryeo increased its borders to Chung-cheon river and Youngheong ( 936 ~ 943 ) and the Amnok River ( 933 ) .