"You were a young man then."
Translation:Tehdy jsi byl mladý muž.
"Byls tehdy mladý muž." is much more natural. I cannot imagine your answer without a stop and starting a new sentence. "Byls mladý muž. Tehdy..."
I want to upvote this question. What's the deal with 'byls' and 'bylas'? When do you use 'jsi byl(a)' and when do you use 'byl(a)s'?
"byls" is equivalent to "byl jsi" if you need the "jsi byl" word order, it is better to stay with these two words. Or sometimes append the s clitic to the preceding word instead, but it is more difficult to do it correctly. You cannot append it to the and of "byl" when the "jsi" is before it and not after.