（1）当主语是以aim, suggestion, duty, way, hope, idea, intention, purpose, plan, ambition等为中心的名词词组或what 引导的名词性从句时，其后常用动词不定式作表语，说明主语的内容。
Our plan is to make better use of these materials.
我们的计划是更好地利用这些材料。
What I want to suggest is to start work at once.
我的建议是立刻开始工作。
The best way to encourage your kids to enjoy meals is to involve them in the planning and preparing. 鼓励你的孩子们享受食物的最好方法是让他们参与计划和做饭的准备工作。
（2）表示主语的具体内容、目的。
The next step is to make sure that you know exactly what is required.
下一步是确保你确切地知道你需要什么。
