I'm going to offer a limited correction that focuses on grammar without changing too much of your sentences. Some of these sentences still might not be perfectly natural, but they're grammatically correct and very similar to what you wrote: * * * Pollution is a problem that all countries of the world suffer from. We destroyed the environment with our factories, and we have put our planet in danger. Some causes are: cars, which launch toxic fumes everywhere; trash, which has been thrown everywhere; and the construction of new cities, which reduces available land for agriculture. Man is the one who has caused this pollution and who will suffer the consequences. These consequences include diseases such as lung and skin cancers that can affect anyone, the disappearance of animals and plants that cause an imbalance in nature, as well as impure air and other consequences. We must correct our mistakes before pollution reaches even higher levels. We have to take care of the environment, and she'll take care of us.

Round two:

 

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Long ago, primitive people mostly lived together to survive, struggling to find enough food to eat and materials to make things to wear. Ancient lives were so simple that the relationship between people was much less complex. However, today, due to highly developed technology, the world is smaller and people have more ways to interact with one another, which leads to the greater role of cooperation. There are two specific reasons below:

First, the bigger the society is, the more problems people have. Due to globalization, not only has the whole population experienced its advantages, but they also have many new problems, such as environmental problems or wars between countries and races, which have crucial effects on people's lives. For example, if there had not been an industrial revolution, the world would not have such serious issues with pollution, with the entire world working to reverse its effects. In today's developed society, people need to work together more to solve problems. Many international organizations, which are made up of many countries, make efforts to save the earth, and fortunately, they have worked in many ways.

Second, thanks to advances in technology, people have more opportunities to make things better. In the old days, due to a lack of good transportation systems, people had greater difficulties in transporting resources from one place to another. This prevented people from making quality goods and having better lives. However, these days, due to convenient transportation systems, people can cooperate extensively with each other. For example, when Apple wants to make smart phones, it does not do it by itself. It needs displays from LG, semiconductors from Samsung and production from factories in China, which requires significant cooperation but is beneficial for all four entities.

In summary, today, people cannot live alone. Due to technology and globalisation, people have more chances to interact with each other, which eventually requires cooperation.

First off, your English is awesome. Can't believe you can write about this type of stuff. Very cool. I'm going to correct this in two ways. First, I'll only make minimal corrections where the grammar or spelling is incorrect (I'll use red text and strikethroughs for this part). Next, I'll rewrite a couple sentences significantly to make the whole story flow better (not going to use editing techniques). First one:

 

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Long ago, primitive people mostly lived together mostly to survive, struggling to find enough food to eat and things to wear. Ancient lives were so simple that the relationship between people was not very important. However, today, due to highly developed technology, the worlds are is smaller and people have more chances to relate with to one another, which leads to the greater role of cooperation . There are two specific reasons below.

First, the bigger the society is, the more problems people get have. Due to globalization, not only does the whole population in the world does have advantages, but also they they also have many problems, such as environmental problems or wars between countries and racials races, which leads to have crucial effects on people's life lives. For example, if there had not been an industrial revolution, people the planet now would not have such serious pollutions, with which all the people on earth are trying to get rid of. However, in this developed society, people need to help each other more more help each other to solve problems. Many international organizations exist which that includes many countries, and they make efforts to save the earth. and fFortunately it the efforts really works in many ways.

Secondly, thanks to high technology, people have more chances to make things better. In the Oold days, due to lack of good transportation systems, people had greater difficulties in transporting resources, which did not allow people to make quality goods and have a better life. However, these days, through convinient convenient transportation systems, people can deeply cooperate with each other. For example, when Apple wants to make smart phones, it can not  cannot do it for itself. It needs LG's displays, Samsung's semiconductors and factories in China with well organized cooperation, which is beneficial for all of three (four?) entities.
To sum up, today, people cannot live alone. Due to technology and globalisation, people have more chances to face each other, which eventually requires cooperation.

Michael's answers seem to be on the mark, so I'll just give a response based on artificial dialogues in Question/Answer form: 1. I think they will. Q: Do you think John and Sarah will come to the party tonight? A: Yes, I think they will. Reasoning: The party is definitely happening, and the person answering the question believes that John and Sarah both are able to attend the party, and will choose to do so. 2. I think they would. Q: If I had a party tonight, will John and Sarah come? A: Yes, I think they would. Reasoning: The party isn't definite, but the person answering thinks that if there was a party, John and Sarah would choose to attend. 3./4. I think they may/might. Q: Will John and Sarah come to my party tonight? // If I had a party, would John and Sarah come? A: I think they may/might. Reasoning: May/might are used when the person has the capability to do something, but their intention is unclear. This is very similar to saying "I think they will come." However, "will" implies a greater likelihood than "may/might." 5. I think they can. Q: Are John and Sarah able to come to the party? A: Yes, I think they can. Reasoning: This assumes that the party is definitely happening, and asks whether the subject is able to join. This doesn't suggest anything about intention - simply, can they, or can they not? 6. I think they could. Q: If I had a party, would John and Sarah be able to make it? A: Yes, I think they could. Reasoning: In this situation, the party might happen or might not happen. The question is asking whether the subject could attend if they party does happen.