I believe theyone cannot fully understand other people's pain, for two reasons.

They is incorrect here because we don't know who 'they' are. 'One' is better here. It refers to mankind/people in general. One could also use 'we' as well.

First, though it seems that human beings can empathize to with one anothers , every single creature, including human beings, naturally think about themselvesitself first.

So t They are just trying to learn what others think, and it is far fromeven they don't really understanding it.

What you wrote was grammatically fine but just isn't the way we say that in English

Second, if you were able to find out what people are feeling, there would be no problems in the world, because theyyou would guess what others or enemies think, and theyyou would not be able to disagree with them, let alone attack them.

(This is a problem of thequestion on Tokyo Uni's 2011 entrance exam of Tokyo Uni.)

I believe theyone cannot fully understand other people's pain, for two reasons.

Referring to unspecified people, in a general sense, "one" is usually best.

First, though it seems that human beings can empathize towith others, every singleall creatures, including human beings, naturally think about themselves first.

The singular "it" could also work, grammatically speaking, but with the next sentence, "they" is better.

So while they are just trying to learn what others think, and it isthey are far from understanding it.

These conjunctions makes more sense to me with the previous sentence.

Second, if you were able to find out what people are feelingfelt, there would be no problem in the world, because they would guessknow what others o, even their enemies, think, and they would not be able to disagree with them, let alone attack them.

(This is a problemquestion ofn the 2011 entrance exam of Tokyo Uni.)

"Problem" is more for quantitative questions, like "math problems," "chemistry problems," and "physics problems."

I believe that peopley cannot fully understand other people's' pain, for two reasons.

First, though it may seems that human beings can empathize towith others, every single creature, including human being,all creatures naturally think about themselves first.

So they areThey just trying to learn what others think, and it isare far from understanding ithem.

Second, if you wereone was able to find out what peopleknow what others are feeling, there would be no problems in the world, because theyone would guessknow what others oose people, who might even be their enemies, think, and they. As a result, one would not be able to disagree with them, let alone attack them.

(This is a problem of thefrom Tokyo University's 2011 entrance exam of Tokyo Uni.)

I believe that peopley cannot fully understand other people's pain, for two reasons.

Generally, one should write the subject in the first sentence, even though the response answers the title/question. This is because when someone writes "they", the subject has already been mentioned before in the response. Putting "they" in the first sentence, rather than "people", makes it vague.

First, although it seems that human beings can empathize to others, every single humans and creatures, including human being,people naturally think about themselves first.

SoUltimately, they are just trying to learn what others think, and it is far fromrather than understanding it.

"And it is far" sounds a bit unnatural.

Second, if youpeople were able to find out what people aothers were feeling, there would be no problems in the world, because they would guess what others or enemiesare think,ing and they would not be able to disagree with them, let alone attack them.

(This is a problem ofrom the 2011 entrance exam of Tokyo Uni.)