At certain stages of a project, most staff members are expected to work additional hours when needed.一个项目进展到某些阶段，如有需要大部分员工都得加班。The budget makes allowances for extra staff when needed.预算考虑到可能需要另外招聘员工。She doubles on keyboards when needed but is mainly on vocals.必要时她会兼奏键盘乐器，但主要还是以歌唱为主。He gave an assurance of help when needed.他承诺在需要时提供帮助。