Help me in understend this card.

[whi]Tainted Well[/whi]

[color=gray]My oponent cheat me ?[/color]
When I trigger this action, all later corrupted unit is damaged...

[color=gray]or I good understend ?[/color]
I trigger this action only when I choose... and damage only choose corrupted unit ?

[color=gray]Sorry for my not fluently language... [/color]:/

If you control this card, you trigger its Action only when you choose, but also only in response to a unit becoming corrupted. It does not create a constant effect that damages all future corrupted units.

So you played it correctly and your opponent is wrong.

Keep in mind, this card does not trigger off of [whi]Warpstone Excavation[/whi]