1) If there are two actions that took place in the past (or will take place in the future), and they are absolutely parallel (both started/will start at the same moment and finished/will finish at another, but also the same moment), then we normally use two imperfective aspect verbs: Когда/пока я принимал душ, Наташа читала книгу. - When I taking a shower, Natasha was reading a book.
2) If something happened/will happen at one particular moment (or within a particular period of time) while the second action was/will be performing (in fact, it started/will start earlier, and finished/will finish when the first one had/will have been completed), we normally use the imperfective aspect for the second ('background') action, and the perfective aspect for the first action: Пока я принимал душ, Наташа полила цветы. - When I was reading a book, Natasha watered the flowers. There is a trick here, though: there are some verbs of the imperfected aspect which can describe a completed action. The meaning of the whole sentence is in this case different from that of the sentence where the corresponding perfective aspect verb is used. These verbs are mainly those of motion, also verbs like приходить, уходить; брать, давать; класть, вешать, ставить; надевать, снимать; просыпаться; садиться, ложиться, вставать; подниматься, спускаться etc. Just compare: "он ушел" means that he has left; he isn't here anymore. At the same time, "он уходил" may mean that he left for some time, but came back; so he is here now. In the compound sentences where one of the verbs describe a 'background' action, the difference is even more pronounced: "Пока я принимал душ, приходила Наташа" - I missed her, "Пока я принимал душ, пришла Наташа" - she came and stayed, so she is at my home now, we can have some tea and chat.
Are there any other cases when indefinite aspect verbs describe completed actions? Definitely, yes. For example, verbs like есть, пить, видеть, слышать, смотреть, слушать, знать, писать, читать, звонить (and their synonims) can be used in the context like "Спасибо, я уже обедал сегодня" (declining an invitation for a lunch) or "Я уже смотрел этот фильм, давайте пойдем на что-нибудь другое" (explaining why you don't want to watch the film your friends suggest watching in the cinema).
3) Before describing the next point, I need to make some introductory notes. As you may already know, the verbs "начать" and "кончить" ("закончить") are used with imperfective aspect verbs only: "начать считать", "кончить работать". At the same time, there are some perfective aspect verbs which mean the same, so they can be replaced by "начать/кончить + imperfective aspect verb": засмеяться - начать смеяться, доделать - кончить делать (many verbs with the prefix "до-" are of this kind). So if one action started/finished (will start/finish) while the other lasted (will last), we use the perfective aspect verb for the former one, and the imperfective aspect verb for the second one: "Пока я спал, Наташа домыла (кончила мыть) посуду".
There are also the verbs "начинать" и "кончать" ("заканчивать") which are also used with imperfective aspect verbs. The difference between "начать + imperfective aspect verb" and "начинать + imperfective aspect verb" may be described using the information from the previous post.
There are also the verbs "начинать" и "кончать" ("заканчивать") which are also used with imperfective aspect verbs. The difference between "начать + imperfective aspect verb" and "начинать + imperfective aspect verb" may be described using the information from the previous post.
4) Let's now discuss which verbs are used when one is talking about two actions in the past or future that follows one another, so that there is a sequence of those two. If both actions are supposed to be completed, than we're likely to use two perfective aspect verbs. For example, "Когда/после того как я получу электронное письмо (e-mail), я (сразу же) напишу ответ"; here it's supposed that you will complete both actions up to some moment in the future.
5) When you don't want to emphasize the fact that the second action will be finished up to some moment (for example, if you describe the relaxation after work :)) than you may use the following combination: a perfective aspect verb for the first action and the imperfective aspect verb for the second one: "Когда/после того как я допишу статью, я буду отдыхать". Of course, you may replace the perfective aspect verb by the combination of "(за)кончить" in the required form and the imperfective aspect verb: "Когда я закончу писать статью, я буду отдыхать". These constructions are normally refer to the future. Talking about the past, we normally use the combination of "начал(а, о)" (or - in some cases - "стал (а, о)") with the imperfective aspect verb in the second part: "Когда Наташа дочитала книгу, она начала рисовать".
Now the main cases, when the verbs of different aspect are used, have been described, I hope, so there are only few additional tips to cover in the next post, which is coming soon. Until then you may practice a bit. Hope, the figure below will help you! :)