I tend to disagree due to the presence of a clearly visible genitive case in your solution. Sometimes it is just necessary to be as close to the original as can be in translating, especially if learners are supposed to entertain a certain awareness and eye for those slight grammatical differences. With your solution, another reading would be possible as well: "I buy the birthday cake from grandfather." So the meaning is not exactly all the same. Eventually it will all boil down to didactics in such scenarios. I reckon your translation right in a way, but not the best option. There are "false negatives" around in this Korean section which are waaaaay worse. Here I can at least make a tiny argument in favour of this variant's rejection (be it sufficient or not). Cheers!
that's the problem with this course in my opinion. it flips between intended meaning and literal translation too much and it's confusing.
"I buy (my) grandfather a birthday cake" should be another accepted translation. Literally zero difference in meaning
Yep. And I think it's a better translation. For grandfather would need a different structure in Korean