A good way to remember when to use commas is to break the sentence into two sentences. If one of the two sentences is not complete, then there should not be a comma between them. 

Look at the last sentence I wrote- "If one of the two sentences is not complete, then there should not be a comma between them." Both parts of the sentence could be a sentence on their own. "One of the two sentences is not complete." and "There should not be a comma between them." You see? They can both be sentences by themselves, but I want them to be a part of the same sentence so that it's one idea, so I separate them with a comma and add "if" and "then" to make them sound connected. Does that make sense? :)

Fast food

Fast food...well, today there're a lot of opinions about this item. I'd like to say what I think.

On the one hand, Fast food is really easy for business people (don't use comma here) who don't have enough time even for food purchase (I'm not talking about cooking!). Though there are a lot of books with cooking recipes and enough health food in shops, some people still prefer to buy food in chains (the most popular are M'C Donald's and KFC). Because it is ...faster, you know. For some people it's also cheaper.

On the other hand, Fast food is easy now, but later the fanats can may find out a lot of problems whith their health. (*better to say "the fanats may discover a lot of health problems" even though it is technically correct to say "may discover a lot of problems with their health." "Health problems" sounds more natural*) So it's becoming difficult.

I think (don't use comma here) that the value of health is more important than quantity of pence in your pocket.

They say (don't use comma here) that even though people understand it, they can't stop. Maybe because mince's leftovers appeal less (*better to say, "are less appealing"*) than those delicious doughnuts...

What do you think?

Ну, я думаю, что ты ОЧЕНЬ хорошо говоришь и пишешь на английском!! Отлично!!!

I'm very impressed that you knew how to use the phrase "One the one hand/on the other hand!" Very good use of a common English phrase!

You are ready to learn some advanced English grammar- In Russian, after every change in a subject, you use a comma: "Я думаю, что..." или "он не знает, если..."

But in English this isn't necessary. There is not always a comma when there is a subject added to the sentence. So you should say, "I think that he is funny." NOT "I think that, he is funny." There should not be a comma in that sentence, because in English the word "that" isn't necessary (but it's okay to use) so the sentence can be "I think he is funny" - it's all the same idea, so there is no comma. It's hard to remember at first, but I THINK you will understand it easily ;) Молодец!! Если ты хочешь спрашивать мне что-то, я здесь! ))