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Please read and only list the numbers of the correct sentences:

1.I ate dinner. Afterward, I took a walk.
2.I ate dinner. Afterward I took a walk.
3.I ate dinner. I took a walk, afterward.
4.I ate dinner. I took a walk afterward.
5.I ate dinner. I, afterward, took a walk.
6.I ate dinner. I afterward took a walk.
7.Tom didn’t study. Therefore, he failed the test.
8.Tom didn’t study. Therefore he failed the test.
9.Tom didn’t study. He failed the test, therefore.
10.Tom didn’t study. He failed the test therefore.
11.Tom didn’t study. He, therefore, failed the test.
12.Tom didn’t study. He therefore failed the test.
13.I ate dinner ;afterward, I took a walk.
14.I ate dinner; afterward I took a walk.
15.I ate dinner; I took a walk, afterward.
16.I ate dinner; I took a walk afterward.
17.I ate dinner; I, afterward, took a walk.
18.I ate dinner; I afterward took a walk.
19.Tom didn’t study; therefore, he failed the test.
20.Tom didn’t study; therefore he failed the test.
21.Tom didn’t study; he failed the test, therefore.
22.Tom didn’t study; he failed the test therefore.
23.Tom didn’t study; he, therefore, failed the test.
24.Tom didn’t study; he therefore failed the test.

Quoc
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In direct conflict with the anonymous poster above, in the U.S., we say "afterward" not "afterwards."

I don't have the engery to go through this very long list, but constuctions like this: Tom didn’t study. He, therefore, failed the test. don't need the commas around therefore. Also, you would not say "He failed the test therefore, but you would be okay saying "Therefore, he failed the test." And many would say the comma after therefore in that one isn't needed either.
Comments  
afterwards, not afterward.

These look ok to me: 1, 4, 7,8,12,20,24.
 BarbaraPA's reply was promoted to an answer.