The Polish language started to change after the baptism of Poland, bringing in words from Latin such as kościół ("church", from L. castellum, meaning "castle"), anioł ("angel", from L. angelus), often via the Czech language, which also influenced Polish in that era (hence Lechitic "wiesiełyj" yielded modern Polish "wesoły" and "wesele"). Also, in later centuries, with the onset of cities founded on German law (namely, the so-called Magdeburg law), Middle High German urban and legal words filtered into Old Polish. Around the 14th or 15th centuries the aorist and imperfect became obsolete. In the 15th century the dual fell into disuse except for a few fixed expressions (adages, sayings). In relation to most other European languages, though, the differences between Old and Modern Polish are comparatively slight; the Polish language is somewhat conservative relative to other Slavic languages.
The difficulty the medieval scribes had to face was attempting to codify the language was the inadequacy of the Latin alphabet to some sounds of the Polish language, for example cz, sz. Thus, Old Polish does not have a standard spelling. One letter could give several sounds - e.g. s can be read as s, sz or ś. Writing words was almost entirely consistent with the spelling of Latin, for example. Bichek - Byczek, Gneuos - Gniewosz etc.
The Book of Henryków (Polish: Księga henrykowska, Latin: Liber fundationis claustri Sancte Marie Virginis in Heinrichau), contains the earliest known sentence written in the Polish language: Day, ut ia pobrusa, a ti poziwai (pronounced originally as: Daj, uć ja pobrusza, a ti pocziwaj, modern Polish: Daj, niech ja pomielę, a ty odpoczywaj or Pozwól, że ja będę mełł, a ty odpocznij, English: Come, let me grind, and you take a rest), written around 1270.
About 1440 Jakub Parkoszowic, a professor of Jagiellonian University, tried to codify the Polish alphabet. He wrote the first tract on Polish orthographic rules (in Latin) and rhyme Obiecado (in Polish). The reform consisted in the introduction of round and unrounded letters on the distinction between hard and soft consonants. It also contained merging double vowels to a long vowel, for example: aa - /aː/. Parkoszowic's proposal was not adopted, and his theoretical concepts had no followers.
Over the centuries Old Polish pronunciation was subjected to numerous modifications. These are only the most basic ones.
The consonant system transferred into the soft coronal consonants, for example /tʲ, dʲ, sʲ, zʲ/ for /t͡ɕ, d͡ʑ, ɕ, ʑ/. Many consonant clusters were simplified; which in the process made assimilation reverse many consonants in words, before that following a voiceless consonant.
(The introduction to The Legend of Saint Alexius (15th century)