"Love to see this happen?" or 'Love to see this happens?" What's the grammar rule behind this? [closed]

Ignoring the lack of a subject, both OP's examples are syntactically fine. Note that

1: He wants to see it happen
means
seeing it happen is something [that] he wants to do,

...but...

2: He wants to see [that] it happens
means
the fact of it it happening is something he wants to ensure comes to pass

Hence although it's a rather unlikely thing to say, if we knew it was from a native speaker we could reasonably interpret OP's (Love to see it happens) as meaning I would really like to be tasked with making sure that this takes place. In practice though, if it was from a learner, it would simply be seen as a mistake. He almost certainly intended the first meaning above, so it should be ...see it happen.

The relativizer "that" is optional in #2 above, where to see [that] X happens / takes place / is true means to take such actions as are necessary to bring about some desired outcome (X occurring, in this case). It's related to the idiomatic usage...

see to (someone or something)
To care for, attend to, or deal with someone or something _{(thefreedictionary)}

Have a look through results for see it doesn't happen in Google Books for similar examples.