> what is the difference in the meaning when we use мочь и хотеть+ perfective and imperfective > 1)_я могу покупать продукты. > _я могу купить продукты. The first one is an statement about your "general" ability to buy something. I can hardly think of a situation where I'd say something like that. The second one sounds like an suggestion. "If you want/don't mind, I could do that". The difference between these verb is that "купить" is a single act while "покупать" implies many acts of shopping over time. > 2)_я хочу есть. > _я хочу съесть. есть - eat. съесть - eat something up. Basically, "Я хочу есть" means "I'm hungry". "Я хочу съесть" is incorrect because you need an object you want to eat up. > and is it right that when we use verb хотеть in the negation we always use it with imperfective or what ? > because I see in textbook they wrote just (НЕ+ Несовершенный) and they didn't wrote that use negation with perfective( Не+Совершенный)? > like that _Я не хочу писать письма. or can we say Я не хочу написать письма. "Я не хочу написать" is incorrect. Хочу is an imperfective verb, and написать is a perfective, so they clash. You need put "хочу" in the perfective aspect, i.e. "захочу". "Я никогда не захочу ...". Something like that.

Just to sum up (Андрей gave a very good explanation of most things): 1) Хотеть / нужно / можно + Perf puts the accent to the result/action that happens once. - Мне нужно прочитать эту книгу (до завтра/быстро и т.д.) 2) Хотеть / нужно / можно + Imperf means you're talking about habits, sth that happens regularly. - Мне нужно читать новости по-русски каждый день. 3) In negative sentences we never use Perfective (the only exception is Imperative, but that's a particular case). Compare: - Я хочу прочитать эту новость. - Я не хочу читать эту новость. (не хочу даже начинать процесс)

There is the big difference between these verbs. Мочь - can ; to be able to do. Хотеть - to want; to wish. For example: if a person can do smth., it is not means he want to do this. In turn, if a person wants to do smth., it is not means he will be able to do this. In whole, all depends on the sentences. you must learn every particular way. about your questions: 1. both of phrases are possible. They mean a person will be able to buy ( you have enough money, time, etc.) 2. both of these phrases good. But these phrases don't explain, a person can eat or cannot.

the difference is situational... "я могу покупать продукты" is less common than "я могу купить продукты"... 2)_я хочу есть = i'm hungry, я хочу съесть = i want to eat (not drink or bite)