BYĆ - PRESENT DECLENSION
jestem - I am
jesteś - you one are
jest - he is, she is, it is
jesteśmy - we are
jesteście - you many are
są - they are

PREPOSITIONS AND CASES
In the first dialog you have learned the prepositions:
w + local - in, do + genitive - to, na + accusative - on, z (or s-) + genitive - from
In Polish words have different endings depending on a case, for example:
praca - job (nominative case), pracy - job (genitive case, local case), do pracy - to job
In future you shall learn how to create cases.
Polish has 3 main types of prepositions:

prepositions with GENITIVE, ACCUSATIVE and INSTRUMENTAL:
an example of such preposition can be: "nad" - above