The noun 兵 (bīng) means "soldier" or "military personnel." It can appear on its own or as part of compound words related to military service. For example, 我是一个兵 means "I am a soldier."
他当了三年兵,学到了很多东西。
tā dāngle sān nián bīng, xuédàole hěn duō dōngxi.
He served as a soldier for three years and learned a lot.
边境上的兵日夜守卫着国家的安全。
Biānjìng shàng de bīng rìyè shǒuwèi zhe guójiā de ānquán.
The soldiers on the border guard the nation's security day and night.
我爷爷年轻的时候也当过兵。
Wǒ yéye niánqīng de shíhou yě dāng guò bīng.
My grandfather also served as a soldier when he was young.