Defective verbs have roots ending in a weak letter (و or ي). For regular Form I verbs, the passive participle (the object receiving the action, like "broken" or "written") follows the pattern مَفْعول. When the root ends in a weak letter, the end of the word changes slightly to make pronunciation flow more naturally.
Roots Ending in و
If the root ends in و (like د-ع-و for "to invite"), the passive participle ends in ـُوّ. For example, "invited" is مَدْعُوّ.
Roots Ending in ي
If the root ends in ي, the passive participle ends in ـِيّ. The vowel before it changes to a kasra to match the ي. For example, the root for "to grill" is ش-و-ي, making the passive participle مَشْوِيّ (grilled).
Similarly, "fried" (from ق-ل-ي) is مَقْلِيّ:
Other everyday examples include "built" (مَبْنِيّ), "folded" (مَطْوِيّ), "forgotten" (مَنْسِيّ), and "thrown" (مَرْمِيّ).
Agreement as Adjectives
Like all participles in Arabic, passive participles frequently act as adjectives. They must agree with the noun they describe in gender, definiteness, number, and case.
To make a passive participle ending in ـِيّ feminine, simply add a ة, which results in the ending ـِيَّة.
Notice how the feminine form is used for grammatically feminine nouns (like شواية for grill) as well as non-human plural nouns (like بطاطس for potatoes).