2 sentences for 跟上 (gēn​shàng — catch up)

1. 他跑得太快了，我很难跟上。

   tā pǎo de tài kuài le, wǒ hěn nán gēn​shàng.

   He runs too fast; it's hard for me to keep up.

   Source: Dong Chinese Dictionary

2. 他的思维非常跳跃，别人很难跟上他的逻辑。

   tā de sī​wéi fēi​cháng tiào​yuè, bié​ren hěn nán gēn​shàng tā de luó​ji.

   His thinking is very jumpy, making it hard for others to follow his logic.

   Source: Dong Chinese Dictionary