"When I sleep, the cat sleeps."
Translation:Khi tôi ngủ thì con mèo ngủ.
I disagree with this. It is saying you must use then. The "then" used in the English translation of this indicates that this is the only time the cat sleeps. Tell the cat that.
The translation means the same thing, it's just that Vietnamese uses a different word construction. You can omit the word "thì" and it would mean the same thing. If you wanna say "I sleep (first) then the cat sleeps" you'd say something like "Tôi ngủ rồi con mèo mới ngủ".