I think nothing indicates that "there" should be used, but in English "there are" is just the proper (or at least best) expression to inform someone about rats in the grass. As I understand the rats (or mice) would be subject of the sentence if its purpose was to inform about the whereabouts of certain rats.
"हैं" can also translate as "there are". Other usage of the adverb "there" in Hindi are: वहाँ there, thither, therein, yon, yonder, thereat उधर there, thither, thitherward, therein, thereto उस स्थान पर there
Yeah, this question says that's a wrong answer (I just put 'in the grass are rats').
That's probably because it's a dated form of writing. It's in literary works that you find such constructions nowadays.
The problem here is the word order, but as it is loose in Hindi, they are also possible.
how is ''in the grass there are rats'' wrong? (any spelling errors not considered) I think it should be accepted.
It gave me "the grass has rats" as the corrected translation. I don't think you would ever say that in English?