"I am unhappily in love with a happily married woman."
Translation:Jsem nešťastně zamilovaný do šťastně vdané ženy.
I don't think so
"s/se" is not the right preposition in Czech.
She is "šťastně vdaná žena" = "happily married woman", not "šťastná vdaná žena" = "happy married woman", that is quite a difference.
I tried to use it because the English translation used “with”. How am I supposed to know to use “do” which usually translates to “into”?
Various verbs or other expressions use different prepositions in different languages. You have to remember them, you can't translate prepositions separately. "in love with someone"="zamilovaný do někoho"