2 sentences

1. 他是这支篮球队的主力后卫。

   tā shì zhè zhī lán​qiú​duì de zhǔ​lì hòu​wèi.

   He is the starting guard of this basketball team.

   Source: Dong Chinese Dictionary

2. 主力部队撤离时，留下了一个营担任后卫。

   zhǔ​lì bù​duì chè​lí shí, liú​xià le yī gè yíng dān​rèn hòu​wèi.

   When the main force withdrew, a battalion was left behind to act as a rearguard.

   Source: Dong Chinese Dictionary