From every Arabic verb, you can instantly create a noun meaning "the one who does it." This is called the Active Participle (اسم الفاعل — Ism al-Fa'il). It appears thousands of times in the Quran!
The Formula — فَاعِلٌ Pattern
Root (كَتَبَ)
ك - ت - ب
3 root letters
Apply فَاعِلٌ pattern
كَاتِبٌ
Kaatibun = A Writer
The Pattern: Take the 3 root letters. Put an Alif (ا) after the 1st letter. Put a Kasra (ِ) under the 2nd letter. Add ـٌ at the end → فَاعِلٌ
Common Active Participles — You'll See These Everywhere!