Formal:
* 弟弟 (dìdi): This is the most common and general term for "little brother." It is used for siblings and also for younger male cousins.
* 幼弟 (yòudì): This is a more formal term for "little brother," often used in written language or when addressing someone older or of higher status.
Informal:
* 小弟 (xiǎodì): This is a more informal term for "little brother," often used by someone younger or of lower status when addressing someone older. It can also be used to show respect or humility.
* 小弟弟 (xiǎo dìdi): This is a very informal term for "little brother," often used when speaking to or about a very young brother or when using baby talk.
* 弟弟 (dìdi) + nickname: You can also use the general term "弟弟 (dìdi)" followed by a nickname for your brother, like "弟弟 (dìdi) 小明 (xiǎo míng)."
Choosing the right term:
* Consider the age difference between you and your brother.
* Consider the formality of the situation.
* Consider the relationship between the speakers.
For example, if you are speaking to your older brother, you might use "小弟 (xiǎodì)." If you are writing a formal letter to a distant relative, you might use "幼弟 (yòudì)." If you are speaking to your younger brother informally, you might use "弟弟 (dìdi)" or "弟弟 (dìdi) + nickname."