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Hi. Please check the following. I think numbers 1, 4, 5, 8, 9, 10, and 11 are correct. As for numbers 9 and 10, do you think we should maintain the adverb (or is it?) "so" in the answers too? Thank you in advance.

1. He want to be good at math so he studies math every day.
2. He want to be good at math, He therefore studies math every day.
3. I went to my favorite group's show. Their stage name is "XXX." I had wanted to go to the show since last year. I had a great time today.
4. My first class begain at 9. It had been math, but today it was English.
5. We played soccer today. We gave up the first goal. Then the table had turned around in a moment's time, and we started to make shots. We won finally.
6. He went home by his academy's bus.
7. (dialogue) Who was the girl? It was Jane Doe.
8. (dialogue) Who was the girl? She was Jane Doe.
9. (dialogue) Why does he study so hard? He studies very hard so he could get good grades in school.
10. (dialogue) Why does he study so hard? He studies very hard to get good grades in school.
11. My friends bought some delicious foods for me. They were chicken and pizza.
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As for numbers 9 and 10, do you think we should maintain the adverb (or is it?) "so" in the answers too? -- No, it is not necessary to do so. I have underlined problems in the sentences below and corrected #11 for you. The others are acceptable.

1. He want to be good at math so he studies math every day.
2. He want to be good at math, He therefore studies math every day.

4. My first class begain at 9. It had been math, but today it was English.
5. We played soccer today. We gave up the first goal. Then the table had turned around in a moment's time, and we started to make shots. We won finally.
6. He went home by his academy's bus.

9. (dialogue) Why does he study so hard? He studies very hard so he could get good grades in school.

11. My friends bought some delicious chicken and pizza for me.
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Hi. Thank you. As for no. 11, why would the sentence I wrote would be wrong? I think this would be correct: His favorite foods are chicken and pizza. Also as for nos 1 and 2, what would be wrong to study math every day to get better at math? Thank you in advance.
As for #1 and #2 and the other sentences where I have indicated problems, please post your corrections.
As for #11, yours consisted of two overly simple sentences that would be composed by a child. I have made it an adult sentence.