"И" means exactly "and", emphasises two (or more) pieces of information together. Example: Я поеду в Москву и в Санкт-Петербург. "А" has no exact translation into English, it is mostly translated as ""but" (sometimes as "and") and is used for 2 situations: - With particle "НЕ": for more accurate description of the situation or action. Example: Я поеду в Москву, А НЕ в Санкт-Петербург. (or Я поеду НЕ в Москву, А в Санкт-Петербург.) - With different subjects: for more accurate emphasis of the subject. Example: Я поеду в Москву, А ТЫ поедешь в Санкт-Петербург. "A" should also be differentiated from "НО", which means exactly "but" and expresses opposition. Example: Я поеду в Москву, но не в Санкт-Петербург. - Unlike "A", it demonstrates a pricipial opposition to the first part of the sentence. One more example: Я поеду в Москву, но ты поедешь в Санкт-Петербург. - (I'll go to Moscow, but you (must) go to St. Petersburg.)

И- it is used for adding Я заказал рыбу и мясо. i ordered fish and meat. А- it is used for opposition. Я заказал рыбу, а мясо не стал. i ordered fish, and meat did not do. it is to answer simply

thank you very much mila!

Mostly the conjunction "и" serves to connect words,phrases or sentences.The conjunction "а" has more syntactical and grammar meanings and functions but very frequently it`s used to show the opposition or division.This conjunction is always preceded by a comma.