The sound masculine plural (جمع المذكر السالم) is one of the simpler plural forms in Arabic. It's formed by adding the suffix ـونَ (in the nominative case) or ـينَ (in the accusative and genitive cases) to the singular masculine form. The word "sound" (سالم) means the singular form remains "intact" — its root letters and vowel pattern are preserved.
This plural is typically used for masculine human beings described by adjectives or participles. For example: مسلمٌ (Muslim, singular) → مسلمونَ/مسلمينَ (Muslims, plural), معلمٌ (teacher) → معلمونَ/معلمينَ (teachers), and مجتهدٌ (diligent) → مجتهدونَ/مجتهدينَ (diligent ones).
Not every masculine noun can take the sound masculine plural. It has specific conditions: the word must be masculine, refer to a rational being (human or angel), and typically be either a proper name, an adjective, or an active/passive participle. Nouns that don't meet these criteria typically form their plurals using broken plural patterns instead.
The dual case endings of the sound masculine plural (ـونَ for مرفوع and ـينَ for both منصوب and مجرور) make it one of the categories that are إعراب'd with letters rather than vowel marks. This places it in the same category as the dual and the five nouns (الأسماء الخمسة) — all parsed using letters. Understanding this pattern simplifies your study of Arabic case endings considerably.