3 sentences

1. 他的脾气古怪，让人难以捉摸。

   tā de pí​qi gǔ​guài, ràng rén nán​yǐ zhuō​mō.

   He has an eccentric temperament that is hard to pin down.

   Source: Dong Chinese Dictionary

2. 他的脾气反复无常，让人难以捉摸。

   tā de pí​qi fǎn​fù​wú​cháng, ràng rén nán​yǐ zhuō​mō.

   His temper is fickle and unpredictable.

   Source: Dong Chinese Dictionary

3. 他的态度非常暧昧，让人难以捉摸他的真实意图。

   tā de tài​du fēi​cháng ài​mèi, ràng rén nán​yǐ zhuō​mō tā de zhēn​shí yì​tú.

   His attitude was very vague, making it difficult to discern his true intentions.

   Source: Dong Chinese Dictionary