When the person wants to use the present continuous, he has to think about how he talking about what he is doing now or in the near future. This explanation does not do very well for students who ask about what to say about what they are going to do next year. I would still use the present future because to me next year is near enough and will use the future simple or the use of going and infinitive to talk about something that is going to happen in the distant future. That depends on what the person wants to relate and readers are invited to look up my explanation on how to discuss events in the future.
Talking about something done "during a period" in the past means that the person can the past continuous. Notice how I have not said anything about talking about something done in the past during a specific time. This tense is used in relation to the past simple tense in order for the learner to understand it's importance. He can also use it on its own but its association to a more recent past allow the listener to know the relationship between two past events and that something was happening when another action occurred. Usually the second action interrupts the first and this is how the Anglophone introduces the fact that there was interplay between two events.
In the example I was reading a book when you called then means that the reading was the first activity that was happening when an interruption, the telephone call occurred. The student can understand this tense best when asked what action happened first before the interruption or vice-versa, what the activity was that interrupted the first.
