I like explaining difficult grammar nuances as simply as possible, so I have developed my own explanation for different aspects of the Russian verbs of motion. Some of my students have found it helpful, so I decided to share it with others. This particular explanation works for the Future tense, but it also can be transferred to the Past with some adjustments.
First, let me remind you that the Russian word for go is идти (on foot). This is an imperfective verb. By adding a prefix you can modify its meaning and make a perfective verb out of it:
- по+идти = пойти (to start moving, to leave)
- при + идти = прийти (to come, to arrive)
Your way to the park starts with leaving a house and ends with arriving to the park. Those two major events can be represented as the vertical lines between you in the present and the part in the timeline:
Any action that can be represented as a vertical, a thin slice of your time line should be translated into Russian as perfective. For the first line, leaving home and starting your way to the park, you may use the word пойти: Сегодня я пойду в парк. (Today, I'm going to the park). For the second line – your arrival to the point of destination – you may use the word прийти: Я приду в парк через час. (I'll come to the park in an hour).
Imagine that you decide to take a walk to the park. You expect the road to take you about one hour. In your timeline, this hour is the space between the two vertical lines. It is not an event that takes only a thin slice of your timeline, it is rather a process. Any action that can not be imagined as a dot in the time line, that has no clear “borders”-vertical lines, requires an imperfective verb. In this case, we have to use an imperfective verb идти. Я буду идти в парк час (It'll take me one hour to reach the park).
It works for other verbs of motion too. For example, лететь (to fly, imperfective):
- В понедельник я полечу в Москву. I'm going to Moscow next Monday.
- Я прилечу в Москву в полдень. I'll arrive to Moscow at noon.
- Я буду лететь в Москву четыре часа. I'll be flying to Moscow 4 hours, the flight will take 4 hours.
I hope my explanation helped you to understand the difference between perfective and imperfective verbs of motion. Please do not hesitate to contact me if you have any questions. Your questions are always welcome!