Questions & Answers about wǒ dìdi míngnián yào shàng dàxué, tā hěn xīngfèn.
Both 我弟弟 and 我的弟弟 are grammatically correct, but there is a nuance.
With close family members and some kinship terms (like 爸爸, 妈妈, 哥哥, 弟弟, 妹妹, 儿子, 女儿), native speakers very often drop 的:
- 我弟弟 = my younger brother
- 我妈妈 = my mom
Adding 的 (我的弟弟) is:
- a bit more formal or detached, or
- used when you want to emphasize “my younger brother (not someone else’s)”.
In everyday speech, 我弟弟 is the most natural choice here. Using 我的弟弟 would not be wrong; it just sounds slightly heavier or more explicit.