Why ?
Firstly in English you can't say you without the helping verb ( to be ) in this case
You're surprised
انت تكون مصدوم
We add the ( ed ) because it's in the past , the action happen and stopped .
You will speak NOT will to speak
Because willing to do something is to make a plan for it, you will do it but you're still thinking
But in this case we are talking about an action that will happen immediately
You won't think and make plans then do it
In an involuntarily way
Because in English الصفة قبل الموصوف
A beautiful car
Black cat ................ Etc!
You're welcome