Rule 1: 1. Four ways of breaking a word
Looking at how the singular is altered, broken plurals fall into four groups: (1) internal change only; (2) internal change plus an ending; (3) internal change plus a prefix; (4) internal change plus both a prefix and an ending.
سَقْفٌ ← سُقُوفٌ
saqfun → suqūfun
a roof → roofs
(1) internal change only.
غُرْفَةٌ ← غُرَفٌ
ghurfatun → ghurafun
a room → rooms
(1) internal change; the ة is lost.
جَارٌ ← جِيرَانٌ
jārun → jīrānun
a neighbour → neighbours
(2) internal change + ending ـَان.
فَقِيرٌ ← فُقَرَاءُ
faqīrun → fuqarāʾu
a poor man → the poor
(2) internal change + ending ـَاء.
قَلَمٌ ← أَقْلَامٌ
qalamun → aqlāmun
a pen → pens
(3) internal change + prefix أَ.
سَرِيرٌ ← أَسِرَّةٌ
sarīrun → asirratun
a bed → beds
(4) prefix أَ + ending ـَة.
طَعَامٌ ← أَطْعِمَةٌ
ṭaʿāmun → aṭʿimatun
a food → foods
(4) prefix + ending.