[Grammar] leave or leaves

[Jon Kay]

Hi there,
I was wondering if someone could please explain to me why we use this phrase "necessitated that he leave the island" instead of " necessitated that he leaves the island"
Many thanks,
Jon

 

[Raymott]

It would be better if you could post a whole sentence in future.
"His behaviour necessitated that he leave the island." This has already occurred, but you couldn't use "left the island", and you can't use "leaves the island" because he's already left.
"Leave" is the subjunctive. "I demand that he leave the island."

https://en.wikipedia.org/wiki/English_subjunctive

 

[Raymott]

"His behaviour necessitated that he left the island"?
OK, if you say so. It sounds weird to me.