Personal pronouns (zaimki) are subclass of nouns which are used to indicate what or whom is the subject of a sentence. Despite their name, they do not have to refer to a person, but are used to indicate inanimate objects and other nouns as the subject of a sentence as well. Many pronouns in both English and Polish have different forms for gender, while others (depending on the subject) do not. In English there are both singular (I, you, he, she, it) and plural (we, you [pl], they) personal pronouns, and this is the same in Polish too.
The singular personal pronouns in Polish are, ja (I), ty (you), on (he), ona (she), and ono (it). The plural Polish pronouns are, my (we), wy (plural 'you'), oni (masculine 'they'), and one (femenine 'they'). Polish also has a formal set of personal pronouns that should be used in conversation with strangers, older people, business associates, and when trying to convey respect; these are, pan (sir or Mr.) and pani (mam or Mrs./Ms.) in the singular, and panowie (gentlemen), panie (ladies), and państwo (ladies and gentlemen [mixed group]) in the plural.
In Polish, personal pronouns are very rarely used, and this is often cited as one of the things that makes learning nominative verbs in Polish much easier. Instead, the pronoun is implied by the verb conjugation that is used in the sentence. This is particularly true of first and second person positions, where you would only say 'ja lubię' (I like), or 'ty lubisz' (you like), if your desired effect was emphasis on the pronoun itself ('I like', and 'you like'). So, for the most part they will be omitted, for example when introducing yourself or others using the verb 'to be'; it's 'jestem Konrad' (I am conrad), not 'ja jestem conrad'.
Personal pronouns can also be used to indicate the object of a sentence, typically appearing at the end of the construction. The difficulty with Polish pronouns is that when things assume this position, they are often required to change in case, and therefore conjugate accordingly. Where the absence of pronouns in the first and second singular and plural positions made life a lot easier when it came to talking about a sentence subject, the necessity of using pronouns in different cases, to indicate the object of a sentence, for example, can make things a lot harder.
There are conjugations for every pronoun in each of the seven Polish cases, and these mostly take entirely different forms to their nominative original (so much so that they are unrecognisable). For example, if I wanted to say the sentence 'I love you', which, containing the transitive verb kocham ('I love'), would require me to place the sentence's object ('you') in the accusative case; I would have to say 'kocham ciebie', not kocham ty, which makes incorrect use of the nominative form of the personal pronoun ty ('you').
Before you start to approach learning the various case change requirements that Polish demands of personal pronouns, it is a good idea to get used to using them confidently in the nominative case. But remember, they aren't always required, and it will sound archaic and over emphatic if you start using 'ja' to prefix first person verbs when it is not required. From then on in, it's best to learn pronoun conjugation for each case separately, as this will give you an idea of how each one is rendered, and when it is appropriate.