Sound Plurals: -ūn/-īn and -āt

Many nouns for people make a sound plural: the singular stays intact and an ending is added.

• Masculine: ـون (-ūn) when the noun is the subject or the predicate: المعلمون مجتهدون (the teachers are hard-working).
• After a verb or a preposition it becomes ـين (-īn): رأيت المسافرين (I saw the travellers), مع الموظفين (with the employees).
• Feminine: ة is replaced by ـات (-āt) in every position: طالبة → طالبات.

Adjectives of origin follow the same rule: مصري → مصريون, مصرية → مصريات. Many words for things also take ـات, even masculine ones: امتحان → امتحانات, حيوان → حيوانات. Look at the words around the gap: a plural adjective, a plural verb or هؤلاء tells you that a plural is needed.