Hur skriver man "förstår ni" på spanska?

Depends if you want to say it in present “¿Entendéis?”or in past with present sense “¿Entendisteis?” 
In Spanish you always put the open and the closing questions marks. And when you speak Spanish, you normally say the past with present sense. Because you’re asking if someone; in this case them, understood something you just said. But the literal translation is with the present. The present it can apply for example if you’re reading a text in class and you ask if they’re following you. “Do you understand what the author wants to say?” - “¿Entendéis (Present) / Entendisteis (Past) lo que el autor quiere decir?”  But it’s correct to say it both ways.