6 sentences

1. 他下定决心要痛改前非。

   tā xià​dìng jué​xīn yào tòng​gǎi​qián​fēi.

   He is determined to turn over a new leaf and make amends for the past.

   Source: Dong Chinese Dictionary

2. 我还没有下定决心。

   wǒ hái méi​yǒu xià​dìng jué​xīn.

   I haven't made up my mind yet.

   Source: Tatoeba

3. 他下定了决心要改掉这个坏习惯。

   tā xià​dìng le jué​xīn yào gǎi​diào zhè gè huài xí​guàn.

   He has made up his mind to get rid of this bad habit.

   Source: Dong Chinese Dictionary

4. 他下定决心要狠狠改掉这个坏习惯。

   tā xià​dìng jué​xīn yào hěn​hěn gǎi​diào zhè gè huài xí​guàn.

   He is determined to kick this bad habit for good.

   Source: Dong Chinese Dictionary

5. 他下定决心，终于克掉了爱迟到的坏毛病。

   tā xià​dìng jué​xīn, zhōng​yú kè diào le ài chí​dào de huài​máo​bìng.

   He made up his mind and finally broke his bad habit of being late.

   Source: Dong Chinese Dictionary

6. 既然已经下定了学习的决心，就不要轻易放弃。

   jì​rán yǐ​jīng xià​dìng le xué​xí de jué​xīn, jiù bù yào qīng​yì fàng​qì.

   Since you have already made up your mind to study, do not give up easily.

   Source: Dong Chinese Dictionary