Understanding the Difference Between -ִים and -ִיִּים in Hebrew Grammar
Ethnonymic terms (such as יהודי (Jewish), צרפתי (French), אנגלי (English)) can serve both as adjectives and as nouns. Their role as nouns is evident when referring to people, for example, "צרפתי, אנגלי ורוסי נפגשו במסעדה" (a common joke setup). The plural form of these words as nouns uses a single י (yod): יהודים (Jews), צרפתים (Frenchmen), אנגלים (Englishmen).
These terms can also be part of a construct state: חייל צרפתי (a French soldier), and in plural 'חיילים צרפתים' (French soldiers). In this structure, they function as nouns. However, they can also serve as adjectives for both people and other nouns, and then they appear in the plural with two יודים (yods): 'חיילים צרפתיים' (French soldiers), 'יין צרפתי' (French wine) – 'יינות צרפתיים' (French wines).
In practice, construct states like 'חיילים צרפתים' (French soldiers), 'חוקרים אנגלים' (English researchers), 'סופרים יהודים' (Jewish authors) are more common, while adjectival forms are reserved for non-human noun phrases, such as 'מנהגים אנגליים' (English customs), 'מאכלים יהודיים' (Jewish foods). According to the rules of Hebrew grammar, when referring to people, both options are valid, while for other nouns, only the -ִיִּים form is correct (יינות צרפתִיִּים not יינות צרפתִים).
Ethnonyms that denote geographic or national affiliation (like 'יהודי', 'צרפתי') are joined by other terms that indicate group affiliation, such as 'חרדי' (Ultra-Orthodox), 'חילוני' (secular), 'מוסלמי' (Muslim). Even in these, there is a distinction between the plural form as a noun – חרדים, חילונים, מוסלמים with a single י, and as an adjective, e.g., 'מוסָדות חרדיים' (Ultra-Orthodox institutions) with two יודים. Conversely, other terms regularly appear in the plural with two יודים even as nouns, such as 'דתיים' (religious), 'מסורתיים' (traditional).
Note: When a combination of a noun and an adjective in the masculine plural appears in the Bible, usually the adjective comes with a single י: "And Midianite traders passed by and they pulled and lifted Joseph out of the pit" (Genesis 37:28); "And behold, two Hebrew men were fighting" (Exodus 2:13); "Before Jewish men" (Jeremiah 39:9); "Their Jewish brethren" (Nehemiah 5:1, similarly 5:8); "And they subdued before them the
inhabitants of the land, the Canaanites" (Nehemiah 9:24). However, it also appears with two יודים: "And Edomite men, servants of his father, were with him" (1 Kings 11:17).