2 sentences

1. 兹事体大，不可不慎。

   zī shì​tǐ dà, bù kě bù​shèn.

   This is a matter of great importance and must be handled with caution.

   Source: Dong Chinese Dictionary

2. 兹事体大，用特向全体成员发函通知。

   zī shì tǐ dà, yòng tè xiàng quán​tǐ chéng​yuán fā​hán tōng​zhī.

   This matter is of great importance; therefore, a formal notice is hereby sent to all members.

   Source: Dong Chinese Dictionary