Writing

Writing
Look at the position of commas in these sentences.
1 Surprisingly,all the students got good marks.
2 I'm doing maths,chemistry,physics and biology.
3 It's sometimes difficult for me to understand him,but I do my best.
4 They're intelligent,hard-working and lively students.
5 It was a difficult problem,I agree.
我只有2点请求：
如果可以的话,做完后,顺便告诉我下为什么这样做?而且这个的意思我也不知道（我很笨的）,最重要的是为什么.