why does it change from kwietniu, and czerwcu to what is presented above in the transition? these surely are the names of the months . they are not in a sentence.
As you've surely noticed by now, in Polish nouns may change grammatical case depending on its function in the sentence. And so does names of the months. „Kwiecień, maj, czerwiec” are their basic nominative forms, while „kwietniu, maju, czerwcu” are locatives.