弟弟用英语用英语怎么说

"弟弟"用英语可以说作 "younger brother"。

其中, "younger" 意为年龄较小的， "brother" 意为兄弟。

英文例句：

My younger brother is seven years old.
(我的弟弟七岁了。)
I have an older sister and a younger brother.
(我有一个姐姐和一个弟弟。)
My younger brother is taller than me.
(我的弟弟比我高。)

综上，弟弟用英语为younger brother.