Вот здесь почитайте внимательно http://www.govoru.net/3627/%D0%BE%D0%B4%D0%BD%D0%BE%D0%BA%D0%BB%D0%B0%D1%81%D1%81%D0%BD%D0%B8%D0%BA%D0%B8-%D0%B1%D0%BB%D0%BE%D0%BA%D0%B8%D1%80%D1%83%D1%8E%D1%82-%D0%BC%D0%BE%D1%88%D0%B5%D0%BD%D0%BD%D0%B8%D0%BA%D0%B8/