Although English largely separates tense and aspect formally, its aspects (neutral, progressive, perfect, progressive perfect, and [in the past tense] habitual) do not correspond very closely to the distinction of perfective vs. imperfective that is found in most languages with aspect. Furthermore, the separation of tense and aspect in English is not maintained rigidly. One instance of this is the alternation, in some forms of English, between sentences such as "Have you eaten?" and "Did you eat?".