"Tell this to your sister."
Translation:यह अपनी बहन को बताओ।
अपनी बहन को यह बोलो is also right though बोलना is perhaps closer to 'to say'.
यह तुम्हारी बहन को बोलो। This was my answer, but I got it wrong. Is this wrong because of तुम्हारी or बोलो..?
I don't know if Duo accepts बोलो or not (it should IMO).
But तुम्हारी is grammatically wrong here though it conveys what you are trying to say. Since this is an imperative statement, तुम is the implicit subject. As a result, you need to use अपनी to refer to it.
Thanks for the clarification. I tried a combination of अपनी and बोलो, and that was turned down too. So I guess Duolingo doesn't accept बोलो in this case.
It is in the oblique case. However, feminine singular nouns don't change form in the oblique case.
It corresponds to the word 'to' in the English sentence.
'अपनी बहन को'= to your sister
Hindi has a reflexive pronoun (अपना/अपनी/अपने ) that you use when you want to refer back to the subject of the sentence. Imperative sentences (used to give commands and make requests) such as 'Tell this to your sister' have 'you' as the implicit subject. So, while saying 'your' which is pointing to the subject of the sentence, you have to use अपनी, instead of the actual Hindi word for 'your' (तेरी/तुम्हरी/आपकी).
The level of formality will be determined by the verb conjugation.
यह अपनी बहन को बता - is in the तू - level of formality
यह अपनी बहन को बताओ - is in the तुम -level of formality
यह अपनी बहन को बताइए - is in the आप -level of formality