2 sentences

1. 这种断言缺乏事实依据。

   zhè zhǒng duàn​yán quē​fá shì​shí yī​jù.

   Such assertions lack factual basis.

   Source: Dong Chinese Dictionary

2. 他断言这件事一定能成功。

   tā duàn​yán zhè jiàn shì yī​dìng néng chéng​gōng.

   He asserted that this matter would surely succeed.

   Source: Dong Chinese Dictionary