留着一头长发用英语可以说成 "to keep long hair"。
其中，"to keep" 是动词，意思是"保留，保持"；而 "long hair" 则是名词短语，意思是"长发"。
这是一个很普遍的说法，可以在日常英语中经常使用。
例句：
"I decided to keep long hair because I think it looks better on me." (我决定留着长发，因为我认为这样看起来更好看。)
"She has been keeping long hair for more than a decade." (她已经留着长发超过十年了。)
"Many men are now choosing to keep long hair, as opposed to a short, clean-cut style." (很多男性现在选择留长发，而不是短发或者干净剪裁的风格)