How to define a 'present action'?

My reason why I ask is, if we can define it, we may explain the Simple
Present tense.
(For the time being, shall we skip the difference between Simple Present and
Present Progressive for a while?)
A present action seems to be very basic knowledge, but it is most puzzling.
We would say that, logically, the present action happens at the present.
Since the present time is moving continually, however, it is totally
uncertain. So, a present action is very hard to define.

Most confusing, 'present time' seems endless, but present action has to have
an end. Therefore, a present time is different to a present action.

How do English native speakers split the nuance? Or does the present action
have nothing to do with Simple Present tense at all?

Shun
englishtense.com

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