Use i if you want to say that you go on foot. If you use any wheeled means of transportation say jecha . I has a lot of other meanings, other than go on foot . The meaning which may be a little confusing is go out, leave . It does not mean that B is going to go on foot.
B can catch a bus or take a taxi, etc.
In the vocabulary section you are also given the information which cases are required by verbs, for example all 'movement verbs' require an instrumental, so you say lecisz samolotem, jad poci giem , etc. Please go back to the texts and find other examples of such verb phrases. I requires an instrumental, so i na piechot and i piechot are both correct. Introduction to the future simple tense of perfective verbs. In this lesson there are three perfective verbs kupi , zobaczy and odwiedzi .