3 sentences for 弟 (dì — I)

1. 愚弟近日安好，请兄放心。

   yú dì jìn​rì ān​hǎo, qǐng xiōng fàng​xīn.

   Your humble younger brother has been well lately; please rest assured, elder brother.

   Source: Dong Chinese Dictionary

2. 贤弟，别来无恙？

   xián dì, bié​lái​wú​yàng?

   My dear friend, how have you been since we last met?

   Source: Dong Chinese Dictionary

3. 他在给前辈的信中，末尾署名为“弟”以示尊重。

   tā zài gěi qián​bèi de xìn zhōng, mò​wěi shǔ​míng wéi“ dì” yǐ shì zūn​zhòng.

   In his letter to his senior, he signed off as 'your younger brother' as a mark of respect.

   Source: Dong Chinese Dictionary