The human brain is awesome.
It functions 24 hours a day from the day we are born
and only stops when we are taking an exam or fall in love.
Question: I’d like to know the last part “fall in love” is correct? Not falling in love?
I think the sentence should be "… it only stops when we are taking an exam or (we are) fall(ing) in love.
Since “we are” is left out, so fall in love becomes “falling in love”.
But I’m not sure if I am right.
I agree. I don’t know how grammarians explain this, but I relate it to the distributive property of algebra:
a⋅(x+y) = a⋅x + a⋅y.
In (English) grammar, if x and y are not of identical form, it is better style to express them as on the right side of the equation rather than with the more terse left side. Strunk and White probably explain this better without algebra. ))