Hello, Yoon Liat! It is not an irregularity. The verb "see" is followed by either an infinitive or by an -ing form.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
InchoateknowledgeI saw him run = you saw the whole of the processI can't disagree with you - I'm not sure. However, I would suggest another interpretation:
I saw him running = only a part of it did I see
J LewisHi Slava,By aiming for the opposite
I don't think the distinction is whether or not you knew his reason for running. I saw him running represents a still photo; I saw him run is a complete film. If you get what I mean...
I saw him running represents a still photo; I saw him run is a complete film.