Men 们 is a suffix that pronounced with neutral tone and is restricted to human nouns and pronouns only. A human noun becomes a plural noun if it takes on this suffix. It is optional. For example:
laoshi-men 老师们 ‘teachers’
xiongdi-men 兄弟们 ‘brothers’
The suffix -men cannot be used with a monosyllabic human noun. For instance:
*zei-men *贼们 ‘thieves’
*bing-men *兵们 ‘soldier’
The suffix -men can occur as a plural marker with pronouns:
wo-men 我们 ‘I-plural=we’
ni-men 你们 ‘you-plural’
ta-men 他/她们 ‘s/he-plural=they’
Reference
Li, C. N., & Thompson, S. A. (1989). Mandarin Chinese: A functional reference grammar (Vol. 3). Univ of California Press.
