2 sentences

1. 行军时，他因为体力不支而落伍了。

   xíng​jūn shí, tā yīn​wèi tǐ​lì bù​zhī ér luò​wǔ le.

   During the march, he fell behind because he lacked the physical strength to keep up.

   Source: Dong Chinese Dictionary

2. 如果不学习新知识，很快就会落伍。

   rú​guǒ bù xué​xí xīn zhī​shi, hěn kuài jiù huì luò​wǔ.

   If you don't learn new things, you will soon fall behind the times.

   Source: Dong Chinese Dictionary