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Can you please tell me why I should use 'enough about what has happened here' instead of 'enough about what is happened here'? Why should I use a present perfect here?
Thank you!
Because we use verb "to have" and not "to be" to form this.
Something happens when you press this button.
Something happened here yesterday, but I'm not sure what.
Something has happened quite recently.
Something had happened just before I walked in the door.
Something is happening right now.
Something was happening as I walked in the door.
Something will happen tomorrow about that.
At noon tomorrow, something will be happening here.
From the most active situation, happening now, through to the least active situation, which had happened some time ago:
"Enough about what is happening here"
"Enough about what was happening here"
"Enough about what has happened here"
"Enough about what had happened here"
What has happened? 
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