We had an 18-lay over in Frankfurt last week from Milan continuing to Chicago. We had a hotel at the airport due to the proximity for easy access, but wanted to go into town for a meal and a walk around in the evening. We reviewed the Metro ticket machine at the airport station for the appropriate return ticket option information, choosing the English option, but not all of the ticket options and information were in English, so we bought what we thought were 2 returns tickets to the main train station and return to the airport station. We went into town, had a nice walk around the river, then headed back to the airport hotel. During the train ride, we were approached by what looked like security guards who were checking tickets. They looked at ours, told us we had the wrong tickets, and told us we owed a fine of 60 euros each. We were rather upset and begged ignorance as new to the city and train ticketing, but they took us off the train and subsequently pulled out a portable credit card reader and charged our card 60 euros each, which we felt very unable to protest or push back. Sad to say, be careful of metro ticketing in Frankfurt.
Sorry to hear this. A one-way ticket would be 6.60 EUR per person, but you could have purchased a Gruppentageskarte for 21.40 EUR to cover both of you round trip (all day really for all public transport between central Frankfurt and the airport).
Single tickets are one-way only, no onward or return travel.
https://www.rmv.de/c/en/tickets/your-ticket/tickets-overview/single-tickets/single-ticket
btw: "Passengers can also use WiFi at all RMV machines with a WiFi sign. As on the S-Bahn and X-buses passengers simply have to turn on WiFi on their smartphone, tablet or notebook, select "WLAN@RMV"as network and confirm the terms of use. If you want, you can use the network for up to seven days without having to log in again. No personal data is evaluated by the RMV." (source)
And yes, travelers shall always inform themselves what tickets they bought. In your case asking other people would have avoid the situation.
All ticket options are available in English. I show people how to use do this all the time. 2 clicks and you are at the appropriate screen.
As already mentioned, a group day ticket was what you needed. Frankfurt does not offer a round trip ticket or a 24 hour ticket. Only 2 options, single tickets and day tickets, which are valid til close of business.
There are lots of online videos showing you how to use the machines, but frankly they are super easy to use in the English version.
Yes, the ticket controllers do not often wear uniforms. It is how they catch people. Otherwise, those without tickets would jump off the train when they got on.