we have something in Arabic's grammer called balances أوزان awzan, search about them it will help you a lot :)
simply,, we have derivatives of verbs,, so from verbs we can get somethings
so what we do is come with the verb in its origin root, adding to it one or two letters and the result will be derivative word indicate for something,,,
for ex. we have : اسم فاعل>>> describe who had made the verb ( قادم ، ذاهب ،مسافر )
& اسم مفعول >>> describe the thing or the verb (مشاهدة)
so اسم فاعل you add أ after the first letter of the root of the verb (must be 3 letter) like ذهب___ ذاهب ،،، قدم____قادم and from the sentence you will get the time of the verb(future,past or present ).
****note: if the root of the verb was >3 letters,,,, you will add mu (مُــــ) as a first letter and break the pre last letter >> سافر .... مُــسافِـــر
_________________________________________
the same thing to اسم مفعول >>>
أكل >> مأكول
فعل >> مفعول
3-21-<< 321 notice the addition of m & o (م & و) ^_^
شرب>> مشروب
فعل >> مفعول
note: ^_^ in this case verb's root with >3 letters>> we add m (مُــ) as first letter and open the pre last letter >>> شاهد .... مُـــشاهَـــدة
I HOPE it is easy now :)