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One of my friends, John, who majored in English, spent lots of money on a dog that he took care of in order to buy some dog food and the dog's decoration because he really loved the dog. Thus, he could not afford to spend money on him because he was a university student who cannot earn a lot of money. The other day, he had an opportunity to study in the United States, but it was difficult for him to go there. That was because of a lack of money to buy an airline ticket. He decided to go there, so he asked me to look after the dog for a moment. After that, he could study English much more, so he could go to a foreign company thanks to the good experience in the United States.
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Your paragraph is not very logical. You talk about your friend's dog, his studies, his trip to the US and his work.That is too much for one paragraph. Each paragraph should have one main point.
How did your friend get to the US if he had no money for airfare?

One of my friends, John, who majored in English, spent lots of money on a dog that he took care of In order to buy He bought some dog food and the dog's decoration collar and leash, because he really loved the dog. Thus, But he could not afford to spend money on him (You just said that he did spend a lot of money on the dog. You contradict yourself.) because he was a university student who cannot earn a lot of money. The other day, he had an opportunity to study in the United States, but it was difficult for him to go there. That was because of a lack of money to buy an airline ticket. He decided to go there, so he asked me to look after the dog for a while moment. After that, he could study English much more, so he could go work for to a foreign company thanks to the good experience in the United States.
Comments  
Thank you so much, AlpheccaStars!! I really appreciated it. I couldn't agree more with your opinion for my writing!! I will try to rewrite them. Thank you again for your help!