[译] 他18岁那年参军了。
[误] He joined the force when he was 18.

[正] He joined the forces when he was 18.
[注] force 的复数 forces 作“军队”、“兵力”解。又如：Superior forces must be concentrated to destroy the enemy. 必须集中优 势兵力歼灭敌人。