understand doing

要看详细放在句子中了。要看它在句中的意思是，明白了将要做某事，还是立刻明白了执行了那个动作。

understand一般接that从句，你说的两种用法都很少见

1.understand sth/doing sth
2.understand what to do/understand how to do sth
3.understand what you say/said
4.understand that.....
you understand?

to do对