"This boy is with his friend."
Translation:यह लड़का अपने दोस्त के साथ है।
That's because adjectives change in accordance with the case of the noun they qualify. Here, the noun "dost" is in oblique case, and the adjective "apne" reflects that. (Remember that since the word "dost" ends in a consonant, it takes the same form in both direct and oblique case, whereas "apnaa" (direct) is distinct from "apne" (oblique)).
Let me give you a bit different set of sentences to make things clearer:
मेरा बेटा यहाँ है। - My son is here. (बेटा is direct, and so is मेरा.)
मेरे बेटे का खिलौना यहाँ है। - My son's toy is here. (बेटे is oblique, and so is मेरे.)
Sorry for the late response. I wanted to give a proper explanation but didn't get sufficient time, until now. Thanks for bringing up that sentence; the object "her apple" in "She eats her apple" isn't in the oblique case because, as you correctly guessed, it's not an object of a preposition (here, postposition).
Introducing a postposition, however, necessitates using the oblique case.
वह अपना सेब खाती है।
वह अपने सेब को खाती है।
There is a slight difference in the meaning of the two constructions: while the former is used for a general or indefinite object, the latter is used to express definitiveness or emphasize the object and/or the action involving the object. Let me give one more pair of sentences.
(वह) केला यहाँ लाओ। - No postposition -> Object in direct case.
(उस) केले को यहाँ लाओ। - Postposition -> Object in oblique case.
Both of the above sentences mean "Bring the/that banana here", but the second one is referencing a particular banana and/or emphasizing the act of bringing it.
Usually, the postposition is used when the object is a person/people or another animate being(s), and omitted when the object is an inanimate or an abstract object. Omitting the postposition in the animate case can come off as rude, indifferent or just absurd, while using the preposition in the inanimate case may sound excessively pedantic or weird.
I will probably make a post on such usages. It's funny that I didn't notice these nuances before these discussions came up. Let me know if you have more questions.
I'm in my forties and I don't remember ever hearing the word oblique being used. So everyone is talking about oblique since I started learning Hindi just this month. I took Latin in high school. Maybe the word oblique is used in Latin I don't know. Oblique hasn't been mentioned by Duolingo, but everybody is talking about that word. Where did they learn it?
There are daftly hidden lesson notes which introduce it. They're not available in the app, but if you use the web version you can read them by clicking a symbol on each lesson, next to where the 'take test and skip' button is (even on the app), from memory I think it's a key.
The oblique case is sometimes called objective case. I believe in Latin it's split into the dative (the 'motion towards' we've seen so far in Duolingo Hindi lessons) and accusative cases (not sure what Hindi does here, I can't think of an example I've come across yet).
Be sure to read the answers to the first question above with lots more detail. From what I understand there, when the word modified by the pronoun is a direct object of the verb, you use the pronoun "aapna" (for example, if you were translating the sentence, 'He sees his friend.')
But when it is governed by a post/preposition ('ke' in this sentence; 'with his friend'), then it is in what's called the 'oblique case'. So instead you use the oblique form - which in this sentence is "aapne."
Someone please correct me if I am wrong!