In that cases past tense verbs used as imperatives. I.e. when you want to say "Let's go" in Russian, you can say one of these words: - Пойди (ты), пойдите (вы), пойдем (мы); - Пошел (ты), пошли (вы, мы). Actually, when the word Пошел (in this form only) is used as imperative, it is a kind of explicit order and may be little rude. When the sergeant is training his soldiers to jump from a plane with parachute (or without it... :)) he may say "Пошел, пошел, пошел!" (Go, go, go!). If you say someone "Пошел" or "Пошли", it means "Go away!" But if you say "Пошли" and include yourself in this order, it is normal. In fact, when you want to say someone "We have to go now", you often say "Пошли", not "Пойдем". And I can't explain why it happens. Let's say, it is just another unlogical thing in the live language :)

Еxamples of past tense with future meaning (in colloquial speech). 1) A strict order (rude): - Упал - отжался! (Hit the floor - 40 press ups!). Sometimes it's raher a playful indignation. - Орлова, встала и вышла из класса! (Orlova, stand up and go off the classroom!) The past tense stresses that order not to reject, as if it were done already. Comparе: - Пошёл вон! (go about your business! ) - Пошёл ты! ( F.сk you; go to hell) 2) An appeal (призыв) - Пошли. (Let's go.) 3) Аsking if someone minds you leave: - Я пошёл? 4) To inform someone that you will leave immediatley after. - Всё, я побежал ( Всё, я убежал). 2), 3), 4) are used with а verb of motion like полетел, поплыл, поехал. 4) -Так я тебе и поверил! ( An ironic denial of event. One person don't believe in what another one says.) 5) What must happen; что обязательно случится, по мнению говорящего: - Ну всё, я пропал. (пропасть - go to pieces) -Ура, теперь мы точно выиграли!

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Русский и английский языки не очень похожи. Я думаю, что не стоит брать английский язык в качестве образца для изучения русского.