According to Arabic grammar, what is the rule for the predicate when the subject is a non-rational plural, such as اَلْكُتُبُ or اَلْأَبْوَابُ?

Dual and Plural Definite Nouns and the Case-Ending Rule for Non-Rational Plurals

• The predicate may be either feminine singular or feminine plural
• The predicate must always be sound masculine plural
• The predicate must always remain masculine singular