"You were a young man then."
Translation:Tehdy jsi byl mladý muž.
"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.
And when must the "jsi" be before the "byl" instead of after it? I thought it might have something to do with questions instead of statements, but your first remark gives an example of a statement using "byls", so is there another guideline?
Is the auxiliary "jsi" wanting the second position - is that a reason?