Polish has a long and very chaotic history, and continues to evolve at a rapid pace even today.
The Long Era of Old Polish
At one point there was a single language in Europe, which we usually refer to as Proto-Indo-European. At some point in the past, a language known as Proto-Slavic evolved out of this ancestral language, splitting off from other languages that were also evolving. Eventually Proto-Slavic itself branched off into varieties, among them West Slavic, and it is from West Slavic that Polish eventually evolved, being firmly established as its own language by the 9th century. This form of the language is known as Old Polish.
Old Polish remained in force until the 16th century. During this period Poland converted to Christianity, and Latin words began invading the vocabulary, and certain grammatical structures such as the aorist and imperfect forms were dropped completely. What is remarkable is that Old Polish and Modern Polish show comparatively little difference when compared to other languages, such as Old English and Modern English.
The Middle and the Modern
Polish entered into a period of relatively rapid change. Middle Polish, established by the 16th century, only survived into the 18th. The arts, particularly writers such as the poet Jan Kochanowski, had great impact on the language, proposing new rules of grammar and even alphabets that had great influence on the language, explaining some of the speed of change. The result is referred to as New Polish, most commonly, which survived as an era until the 1930s. From that point forward we have what is known as Modern Polish. It’s remarkable how recently so much language history happened!
Modern Polish continues to evolve rapidly, taking on loan words from other languages and cultures and making them semi-permanent with a speed and stability not seen in too many other languages. Some Polish scholars are alarmed by this process and fear a world where their language is unrecognisable in just a few short decades. Others, like me, are simply fascinated by this process. Few languages have shown such flexibility, and the reasons are not entirely clear. Some speculate it has to do with the lengthy period of political instability Poland has undergone, but it is impossible to say with any certainty.