The twentieth century has been a century of momentous alterations of the world. Twentieth century was when the whole globe virtually became connected, and formed a bond as that created by a marriage. Before marriage, the husband and wife are not truly connected. They know their existance, but does not have that much of a influence mutually. However, after their marriage, they are connected, and can not be ignorant to each other. Identical to this, prior to the 20th century, the global world finally knew each other; that is, by the hard work of Christopher Columbus and Magelan. They did have connections, but not much. As the twentieth century came, a new era was announced. The different cultures of the world could no longer have a indifferent, apathical mind. The world was finally globalized. Many modifications were made. In all of this, the most important change was the use of uranium; that is, nuclear energy.
>> 서론에서 말씀하시고자 하는 포인트를 잘 못 찾겠네요. 20세기에 많은 변화가 일어났다. 결혼전에는 서로 알긴 알지만 큰 영향을 못미치던 남녀가 결혼 후에는 서로를 무시할 수 없듯이(= 이 부분 글의 진행을 부드럽게 넘기기 위한 은유적인 표현이라는건 알겠는데, 너무 길어요. 너무 길어요...) 20세기 전에는 콜롬버스에 의해 세계가 마침내 서로를 알게 되었는데, 21세기에는 새시대가 열려서 서로 관심을 가져야만 했다.(= 이 부분 내용 이상해요. 마침내 알게 되었다? 마침내?) 세계는 세계화되었다. 많은 변경이 일어났다. (= 이 부분도 이상해요. 세계화랑 변경이랑 무슨 관계죠? 아무 설명이 없네요.)
To begin with, nuclear weapons changed the whole concept of 'war', from people shooting projectiles and tanks rushing over the hills to a 'press' of a single button. A press of a button can now destroy the whole earth in a matter of minutes. Man's ultimate weapon was developed by a single equation, E=mc2. This weapon proved its force in Hiroshima and Nagasaki, destroying everything in its way. In addition, the survivors suffered greatly by the radioactive rays. My close friend's grandfather was one of the few survivors of the Hiroshima explosion. When I met him and talked about what occured in 1945, he spoke with brevity; "I was working, and suddenly felt light and vibration. That's all I remember." I was also amazed that only two people from his neighborhood is still living. This clearly shows how much nuclear bombs alterated the world, ending World War II.
>> 근거가 힘이 많이 약하네요. "일하는 중이었다. 갑자기 빛과 진동을 느꼈다. 그게 내가 기억하는 전부다."라는 말이 "핵폭탄이 세계를 얼마나 변화시켰는지를 명확하게 보여준다"고 동의할 사람은 별로 없을듯?
Also, nuclear provided nuclear energy, which is becoming an alternative solution for oil energy. There are a lot of alternatives, but only nuclear energy can enable us to acquire the gigantic force we require. Also, nuclear energy is currently the only way for us to explore space, for it gives a massive amount of energy from a small mass. In my country, we are unable to produce oil - not a single drop. However, by constructing many nuclear energy producers; that is, 80% of our need, we are able to procure the amount we need.
>> 우주선이 원자력 추진이었던가요? 그거들이 좀 극단적이라는 느낌이 드네요. "원자력만이 살 길이다" 라는 느낌이네요...
In conclusion, there were many modifications in the twentieth century. However, the develop of atomic energy was the most momentous one, which developed into an alternive energy solution and a ultimate, massive destruction weapon. By these reasons, I conclude that the development of nuclear energy is the thing that should be remembered in the minds of our successors. They must also keep in mind that the force of nuclear energy is a two sided sword, and should use only its virtues, abandoning its vices.
>> 문제는 "20세기에 일어났던 변화 중 기억되어야 할 것이 무엇인가"인데요, 결론은 마치 "원자력을 올바르게 사용해야 한다"처럼 보이네요. 동문서답같은 느낌이랄까요? 마지막 문장(They must also keep in mind that~)만 좀 뺐으면 좋겠어요. 게다가, alternation/modification 이라는 단어를 사용하셨는데요, 이 단어들이 과연 상황에 맞는 단어인지는 좀 생각해 봐야 할 문제같고요. (한국말로는 다 '변화'지만...영어로는 좀 다를듯?)