Rule 1: 1. The pattern فَعَلَ
To talk about the shape of a verb without naming a particular verb, the grammarians use the three radicals of فَعَلَ to do: ف stands for the first radical of any verb, ع for the second and ل for the third. A verb is then said to be "of the فَعَلَ type", "of the يَفْعِلُ type" and so on. Whatever happens to ف ع ل in the pattern happens to the radicals of the real verb.
جَلَسَ
jalasa
he sat: a verb of the فَعَلَ type
Root ج ل س.
سَمِعَ
samiʿa
he heard: a verb of the فَعِلَ type
Root س م ع.
كَبُرَ
kabura
he grew big: a verb of the فَعُلَ type
Root ك ب ر.
يَجْلِسُ
yajlisu
he sits: a form of the يَفْعِلُ type
اِجْلِسْ
ijlis
sit!: a form of the اِفْعِلْ type