3 sentences

1. 山里的老人常告诫小孩，林子里有魅出没。

   shān​lǐ de lǎo​rén cháng gào​jiè xiǎo​hái, lín​zi lǐ yǒu mèi chū​mò.

   Old people in the mountains often warn children that spirits haunt the woods.

   Source: Dong Chinese Dictionary

2. 医生告诫他如果不及时治疗，后果会很严重。

   yī​shēng gào​jiè tā rú​guǒ bù jí​shí zhì​liáo, hòu​guǒ huì hěn yán​zhòng.

   The doctor warned him that the consequences would be serious if he wasn't treated promptly.

   Source: Dong Chinese Dictionary

3. 老师经常告诫我们要珍惜时间，努力学习。

   lǎo​shī jīng​cháng gào​jiè wǒ​men yào zhēn​xī shí​jiān, nǔ​lì xué​xí.

   The teacher often admonishes us to cherish time and study hard.

   Source: Dong Chinese Dictionary