I'm afraid I don't quite understand what you wrote, but if you need more information about the dative case, you might find this helpful:
I suggest you to study the IPA: https://en.wikipedia.org/wiki/International_Phonetic_Alphabet
And the German Phonology: https://en.wikipedia.org/wiki/Standard_German_phonology
JMO, but I think learning and studying the 160 representations in the IPA system will be more technical and work intensive than you are probably looking for. Learning them will require many hours of intensive study and you really need correct audio along with them or you will only learn to pronounce the symbols incorrectly.
You might be better served by doing a pronunciation search on the internet when you have issues with Duolingo's pronunciation (happens often). Just type "[the word] pronunciation" and you will often get a long list of audio and sometimes video pronunciations by native speakers. Forvo.com can be a good source but the examples are not all community verified, many words are not represented, and some represented by only one example; but still worth a try and many here like it.
Should 'yeah' be correct
No; it's considered too informal for this course. Please use the kind of Standard Written English you would use in an essay for school, not the informal language you might use when texting your friends.
Yeah literally comes from the word yes.
And "I'mma" comes from "I am going to", and "wanna" comes from "want to", but those aren't accepted here, either.
Except this course is supposed to be teaching us how to speak a language, not write an essay. I appreciate that teaching the standard form makes sense from a pedagogical point of view, but appealing to the language used in schools is an incredibly weak argument.
Also, the course is teaching us German, not English, and so trying to correct our English as too informal is downright ridiculous - we already speak English, why bother?
Finally, "yeah" and "Imma" are not equivalent linguistic phenomena. "Imma" is a phonetic spelling of a dialectal variant (most dialects do not have "Imma") whereas "yeah" is present across all varieties of English, and is derived from the now archaic form "yea". "Yeah" can be traced back to the 19th century as a word in its own right.
There is an argument to be made that "yeah" isn't always an appropriate translation for "Ja" whereas "yes" is always correct regardless of register, but your argument that "yeah" is always "too informal for this course" is just complete rubbish. There are plenty of examples of accepted translations on the German course which are of just as informal a register as "yeah".
Also, here is a useful tool: