How do you say That's all you need to know in Japanese?

Answer 1

君はそれした知らなければいけない (kimi ha sore shita shiranakereba ikemanai) would mean "That's all you need to know". Literally, this sentence has the meaning "Only that is what you must know". You can also express this sentiment as 君はそれした知らなくてはならない (kimi ha sore shita shiranakute ha naranai). In general, you can use した (shita) + negative to mean "only" and negative ba-form + いけない (ikenai),　ならない (naranai), or だめだ (dame da) to say that you must do something. Literally translated, when you say you must do something in Japanese, you are saying that it is unacceptable to not do so.