In general unstressed О is pronounced as А and stressed is pronounced as О. НЕ is equal to NOT in English НЕТ is equal to NO in English. Also НЕ can be used as an informal shorten variant of НЕТ

"не" is chiefly informal, while you can use "нет" in both formal and informal situations. "O" is pronounced as "a" when it isn't stressed: молокó (malako), корóва (karova), сморóдина (smarodina), etc.

Hi there! You should have given a few examples of your own ir order to make your question clear. Anyway, I will try to answer your question. 1) The letter ''о'' is usually used in writing, but it is pronounced like ''а" in a speech. Written and oral communications are two different aspects of the language that are based on the same rules of grammar, but Russian pronounciation (speech) differs from a written form. Compare: орех (nut) - арех. The first variant is correct in terms of orthography, meanwhie in the second case the vowel "о" is pronunced like ''а" as it is typical to the Russian speech. Thus, you have to know both the correct orthographic and oral forms of words. 2) "Нет" is a denial answer, which measn ''No''. Sometimes it may be used in impersonal sentences like "Нет воды", "Нету ничего" and so on. "Не" is a negative particle. It denies the meaning of a sentence, for instance, " Она не умеет читать" or "Он не очень приятный человек" and so on. If you have more questions, feel free to contact me. Good luck to you in learning Russian!

Билеты будут не сегодня (будут, но не сегодня) Билетов нет (и возможно не будет) - Билеты еще есть? - Не, нету. - Билеты отстались? - к сожалению, нет