Translation:My acquaintance works every Monday.
So "w" takes the accusative case when referring to days ("w każdy poniedziałek"), but the locative case when referring to weeks, months or years ("w każdym tygodniu / miesiącu / roku")...
Is there any part of this language that isn't fiendishly complicated? :)
You can say 'friend' here. 'Przyjaciel' in Polish has a stronger meaning, something like 'best friend'.
What term is used for a Facebook "friend", whom I don't know in real life, and whom I am unlikely ever to meet?
Should there be any difference in pronunciation between kazde and kazdy? (sorry can't put in diacritical marks
Yes, but it's somewhat subtle for English speakers. You could try here: https://translate.google.com/#view=home&op=translate&sl=pl&tl=en&text=Ka%C5%BCde%20Ka%C5%BCdy and press the speaker icon.