Most of Polish verbs have in second person of singular the ending 'sz'. Jesteś is an exception. Jesteś comes from the verb 'być' which means 'to be' and like in English this verb has special coniugation.

you are Polish = jesteś z Polski but you live in Poland/ you are from Poland = mieszkasz w Polsce

The truth is that you need to learn conjugation of basic Polish verbs. It is four verb classes in Polish + irregular verb być The first verb belongs to 3 Class mieszkac Class 3: Verbs in -am –asz, Infinitive in –ać Singular Plural 1st Person -am -amy 2nd Person -asz -acie 3rd Person -a -ają The second one is a Irregular Verb być (be): The verb być (be) is irregular in the present tense: być be jestem I am jesteśmy we are jesteś you are jesteście you are jest he, she, it is są they are Reme,mber also that the verb być is the only verb with a specific future tense: będę I will będziemy we will będziesz you will będziecie you will będzie he, she, it will będą they will Good luck!