3 sentences

1. 他还没有意识到问题的严重性。

   tā hái méi​yǒu yì​shí dào wèn​tí de yán​zhòng​xìng.

   He has not yet realized the seriousness of the problem.

   Source: Dong Chinese Dictionary

2. 政府试图淡化这次事件的严重性。

   zhèng​fǔ shì​tú dàn​huà zhè cì shì​jiàn de yán​zhòng​xìng.

   The government tried to downplay the seriousness of the incident.

   Source: Dong Chinese Dictionary

3. 当他意识到问题的严重性时，恐怕已经为时已晚。

   dāng tā yì​shí dào wèn​tí de yán​zhòng​xìng shí, kǒng​pà yǐ​jīng wéi​shí​yǐ​wǎn.

   By the time he realizes the gravity of the problem, it may already be too late.

   Source: Dong Chinese Dictionary