"इस बिल्ली को गर्मी लग रही है।"

Translation: This cat is feeling hot.

[Carissa789117]

You should let it out of the oven.

[Vivek_Orton]

Why the first word is "iss" instead of "yaga"?

[arielbro]

As far as I understand, the को modifies the noun phrase to an oblique case.

[Ary3117]

Because "This" = "Iss"

[B96lXSTM]

what does को mean in this sentence?

[Ary3117]

To her/it, By it/her, (or like the other person put it "onto" her/it) = The hotness is being felt onto/by/to the cat, therefore "This cat is feeling hot"
"Ko" basically means "she/it IS" as "Usko bhookh lagi hai" = "She is" hungry.

[sid1971]

Ko means as, on, to.....

[Ary3117]

So i see, the Duolingo is trying to flirt with me huh?? ;) ;)

[Anthony101199]

That was the one on the hot tin roof.