How similar is the oblique case in Hindi to the dative case in German?
They seem to be similar, can a Hindi speaker verify or deny this?
I don't know anything about German, but the dative case is what's used for indirect objects, and mostly serves the same role as the preposition "to" or "for" in English (I've read that in German, it's also used for the direct objects of some verbs, but this is a quirk of German and not a general use of the dative in other languages). The oblique case of Hindi doesn't replace the role of prepositions or postpositions (Prepositions and postpositions are the same thing except for whether they come before or after the noun. Hindi uses postpositions, which come after the noun).
The oblique case is used whenever the noun phrase is the object of a postposition. As an example,
"Give the food to my dog"
मेरे कुत्ते को खाना दो
Word by word, the Hindi sentence is "My dog to food give"
In German, I think you'd use the dative case for "my dog" without using a specific word for "to". In the Hindi sentence the word for "to" is translated as को, and because of the postposition को, "my dog" (मेरे कुत्ते) is in the oblique case, similar to how you'd say "to him" and not "to he" in English.
"I will eat with these people"
मैं इन लोगों के साथ खाऊँगी
In German, I don't think you'd use the dative case here, but in Hindi, "these people" (इन लोगों) is in the oblique case because it's the object of "with" (के साथ). This is similar to in English where you'd say "with them" instead of "with they".
From what little I've read about German, nouns don't change in the dative case, but their articles do (is that true?). In Hindi, there aren't articles. Nouns in the oblique case are formed by adding or changing an ending depending on whether the noun is masculine or feminine, whether it has the ending characteristic of its gender, and whether it's singular or plural. Most types of nouns don't change at all in the oblique when singular. Adjectives ending in आ change their ending to one of three endings depending on the case, number, and gender of the noun. Adjectives not ending in आ don't agree with nouns. Like with German's dative, Hindi oblique pronouns have their own forms that don't follow much of a pattern (you just have to learn them).
Lastly in Hindi, nouns don't have a dative, but most pronouns do. However, instead of the pronoun datives, it's also acceptable to use the postposition को (to) with pronouns (in the oblique case of course).
One more note about the oblique case in Hindi: Often, the oblique singular forms are the same as the direct plural forms. This is a common source of confusion on Duolingo. A lot of people see singular oblique forms and translate them as plural, and are confused when the correct translation is singular.