"You were strange then."
Translation:Byls tehdy zvláštní.
By THEN, I mean really strongly stressing it when pronouncing, certainly not the basic way how you would normally read it.
Basically, the fact that he was or is strange would already have to be established in previous communication and this just informs that it was THEN and not at some other time.
Byls zvláštní tehdy. normally does sound strange.
The "-s" postfix is still "jsi", only in a reduced form. And you know we like "jsi" in the second position - so this "-s" will still be in the same position, which effectively means the first word. The notable exception is when we have a reflexive "se" or "si" in the sentence - these take priority in the second position and the postfix "-s" is attached to them, forming "ses" and "sis" respectively.
- Byls tehdy zvláštní.
- Tehdys byl zvláštní.
- Tys byl tehdy zvláštní.
- Ty ses mi zdál zvláštní. (You seemed strange to me.)
Also note that using "-s" is classified as colloquial, but it's very frequent in everyday speech.