The Past Form Alone Is Not Enough

The Six Babs of Basic Triliteral Verbs

Past pattern
Possible imperfect
What to remember

فَعُلَ
يَفْعُلُ
One established pairing

فَعِلَ
يَفْعَلُ یا يَفْعِلُ
Two possibilities

فَعَلَ
يَفْعَلُ، يَفْعِلُ یا يَفْعُلُ
Three possibilities