A volunteer is someone who volunteers to help others.
或者复杂一点：
A volunteer is someone who is willing to help others without being forced to do it.
用定语从句翻译:志愿者就是自愿帮助别人的人
A volunteer is someone who volunteers to help others.
或者复杂一点：
A volunteer is someone who is willing to help others without being forced to do it.