...Of course, we are talking about little amounts of sugar, for example, 1/3 of a bar of chocolate (natural or organic, you know) in a day. Sugar helps the mind to think, in Russia we have a habit - if eat chocolate or something sweet before the exam, you will become very smart. Following that rule there is another rule - if you eat chocolate you will be smart for about 5-10 minutes, then you will become stupid and your mind won't understand any difficult text, for example, a Math exercise. After that I try not to eat many sweets before important tests.
I love sweets, but I'm not a sweet-tooth. Also I try not to eat too many sweet things because of the fat. It's really
hiiiigh-calorieeeee. Here I will suddenly stop, not to make many mistakes :) Thank you for your help in my previous question and have a nice day!

If you have questions about my corrections, feel free to write me here or in private!
Good job by the way! You just need to revise the concept of when to use the articles and when it is not required. 

Why sugar is not bad for your health? [Is this a question or statement?]

...Of course, we are talking about little a small amount of sugar, for example, like 1/3 bar of chocolate (natural or organic, you know) in a day. Sugar helps the mind to think [helps you think],. In Russia we have some the habit - to of eating chocolate or something sweet before the exams and you will be to make you very smart [In Russia, we have the belief that if you eat chocolate or something sweet before exams you will be very smart]. After that rule appears Then there's another rule - if you eat chocolate you will be smart for about 5-10 minutes and then you will become stupid and your mind couldn't won't understand any difficult texts, for example, like Math exercises. After Because of that, I try not to eat many sweets before important tests.
I love sweets, but I'm not don't have a sweet-tooth. Also I try not to eat too much many sweet things because of the fat. It's really
hiiiigh-calorieeeee. Here I will suddenly stop, so as not to make many mistakes :) Thank you for your help in answering my previous question and have a nice day!