C - Instead of "him/her," it should be "them."

• Corrected sentence: If somebody helps you, you should thank them.

A - The sentence should use the present tense: "If you feel stressed out..."

• Corrected sentence: If you feel stressed out, you should take a rest to improve your mood.

C - Instead of "can't want," it should be "don't want."

• Corrected sentence: You should wear a coat when going out if you don't want to catch a cold.

A - Instead of "might be," it should be "should be."

• Corrected sentence: You should be careful when using a knife if you don't want to hurt yourself.

A - The sentence should use the present tense: "If you finish..."

• Corrected sentence: If you finish all your homework, you can go to the football stadium.

D - The verb "suffered" should be in the future tense: "will suffer."

• Corrected sentence: If you stay up late every night, your health will suffer.

A - The sentence is logically incorrect. The correct structure would be: "You must improve your study result if you want to..."

• Corrected sentence: You must improve your study result if you want to stop playing computer games all the time.