为何在英语句子,是TO stay healthy ,而不是Staying healthy?

为何在英语句子,是TO stay healthy ,而不是Staying healthy? 急用：英语作文 How to Stay Healthy 六年级英语作文How to stay healthy how to stay healthy的英语作文 how to stay healthy英语作文急 为什么if you want to stay healthy而不是if you want to stay health how to stay healthy作文 How to stay healthy 作文 Like to stay healthy翻译 how to be stay healthy how to stay healthy是什么意思 stay healthy