Translation:I was working with her brother then.
I wrote “I used to work with her brother”. I noticed that this is never acceptable in any duolingo czech to english module when translating “tehdy”. “Then” is always added. In English this is redundant - hardly anyone would say it.
Well you can also leave it out in the Czech sentence but then you have a different sentence. This one refers to a specific period of time which was probably mentioned earlier.
“I used to work with her brother.” Should be an acceptable translation. The “then” is redundant. In English, “used to” would be the most typical translation for “tehdy”. This course does not accept it in any module without “used to then”. Vladu: not every sentence using tehdy in the course is referring to a specific period of time mentioned earlier.
"Tehdy" refers to some past time which is understood from the context. Explicitly mentioned or not. Tehdy here is a direct counterpart of the "then" in the English sentence. If you wanted to say “I used to work with her brother.” you would say just "Pracovala jsem s jejím bratrem.".
You can link to some other sentence where "tehdy" is used as "used to" instead of "tehdy", and we will see, but I claim it is not as direct translation as the official one. And therefore should not be accepted here.
If “tehdy” refers to a specific period of time, perhaps the meaning would be better conveyed with “at that time” rather than the nebulous “then” or “back then” which still does not refer to a specific time period unless previously specified.
at that time - v té době, toho času
tehdy is really pretty much the same as “then” or “back then”