请教：一个句子中什么情况下开头单词是动名词，什么有时候是动词不定式，什么时候又是动词原形？

Question

这个真的好晕，向高手求教。

Answer 1

动词不定式有逻辑将要的含义，动名词表示既成事实。看着两句话：
To be a teacher is not that easy.
要当老师可不是那么容易的。
Being a teacher is not that easy.
当个老师不是那么容易的。追问

那，表示做过某事用那个嘞？

Answer 2

当动词做主语时,既可用动名词，也可用动词不定式放在句首,祈使句中动词原形放在开头.追问

能不能更详细点。

追答

Playing football is my favourite outdoor activity.
To play football is my favourite outdoor activity.
Come here, please.