As to the verbal aspects, I like the other people's explanations.

But I would advise you something. You are trying to learn all formal rules for the aspects. I am not going to say it is a bad idea, but:
Try asking any Russian "Why do you prefer perfective here and imperfective there?" In 90% of cases they would say: "I do not know why. I just feel it suits here." It is really hard to formalize all the possible nuances of the aspect choice. There is nothing bad in knowing the rules, but you should not think about the rules while speaking!

Read more books in Russian, listen to Russian speech more, and you will gradually start "feeling intuitively" how the aspects are used. Yes, it requires practice, but there's nothing dangerous if you would confuse aspects for some time.

When I started to learn English, it was very hard to me to understand when I should use "a" and "the". Now I know the rules, but I do not think of them when speaking. In many cases, I "feel" which one I have to put.

As to your sentences, I would like to explain some of your logical and grammar mistakes. Hope it would be useful.

Quote Originally Posted by ShakeyX View Post
Ты купила рыбу для ужина сегодня вечером (now I'm using perfective to suggest that this was PRE-DISCUSSED fish, a one time completed action as it is for tonight's dinner, and god knows I want some fish tonight).
Although "рыба для ужина" is grammatically correct, it sounds awkward. Your choice is "рыба на ужин":
- Что у нас сегодня на завтрак? - What do we have for breakfast today?
- Вчера на обед были пельмени. - Yesterday there were pelmeni for lunch.
Compare: "рыба на ужин" (fish for dinner), but "комната для ужина" (a room for dinner) or "скатерть для ужина" (a table-cloth for dinner). Do you feel the difference between "на" and "для"?

"Ты купила рыбу для ужина сегодня вечером?" - "сегодня вечером" is strange.

First, for me it sounds as if you were asking "Did you buy fish for dinner tonight?" i.e. "Did you buy it tonight?". But it implies that "tonight" (сегодняшний вечер) has already passed. Then, it is not clear WHEN this question is asked: "сегодня" has not passed (otherwise, you would use "вчера"), but "сегодняшний вечер" (today's evening) has passed, but it is the last part of the day!
A possible option is "Ты сегодня купила рыбу на ужин?" (if you want to specify "Have you bought it today?"), e.g. she buys a fish for dinner almost everyday, but sometimes she does not. Then this question is logical.

Second, in English you are accustomed to put an adverbial modifier of time to the end of a sentence. In Russian, most often we put it to the begining of a sentence or straight after a subject. As I proposed: "Ты сегодня купила рыбу на ужин?" or "А сегодня ты купила рыбу на ужин?" ("And today, have you bought ...") sounds much more natural than "Ты купила рыбу на ужин сегодня?"

Third, if it was already agreed that she would buy fish, I see no reason to specify "today":
"Ты купила рыбу на ужин?"

Fourth, if you mean "tonight's dinner" rather that "did you buy it tonight", I would rephrase it:
"Ты купила рыбу на сегодняшний ужин?" (Did you buy fish for today's dinner?) Note: "сегодня" includes the whole 24-hour period, Russian does not have an opposition between "today and tonight". Technically, "tonight" is "сегодняшний вечер" (as a noun) or "сегодня вечером" (as an adverb), but you do not need to specify "вечер" when talking about "ужин": everyone knows that "ужин" always assumes "вечером".
But this question (Ты купила рыбу на сегодняшний ужин?) can be logical in some very specific context (cannot think of it right now). Normally, there is no reason to specify that it is "tonight's dinner". It sounds like you oppose "tonight's dinner" (something very special, probably) and all other dinners.

Fifth, the most natural way to ask: "Ты купила рыбу на ужин?" (without specifying time), or even "Ты купила рыбу?" (In real life, we even do not need to mention "dinner").

Quote Originally Posted by ShakeyX View Post
Когда я в России, я буду есть много супов.
This sentence is grammatically incorrect. You are trying to translate "When I am in Russia, I will eat a lot of soups" literally.

Note: In English you use the present tense in a subordiante clause (When I am in Russia, ...), but you mean future.
In Russian, you need future if you mean future: "Когда я буду в России, я буду есть много супов".

BTW, "много супов" normally means "a lot of different kinds of soup". And a more natural way to convey this idea is to say:
"Когдя я буду в России, я буду есть много разных супов" (since "много супов" is a bit unusual for colloquial speech).
If you simply mean "I will eat soup a lot" (not different kinds of it), I would say:
"Когдя я буду в России, я буду часто есть суп."

Quote Originally Posted by ShakeyX View Post
Maybe someone could give me a short story/conversation which use есть and поесть excessively.
The verb "поесть" implies a limited amount within limited time. Same as "попить" (to drink a little), "почитать" (to read for a while), "погулять" (to have a walk), "посмотреть" (to have a look) etc.
The verb "съесть" means "to eat the whole amount of something specified before".
And "есть" is just "to eat" in general.

Perfective verbal prefixes sometimes modify the meaning of imperfective verbs.