As an example of this, native German speakers are often better at knowing when to use "who" vs "whom" because German grammar rules are in some ways a superset of English grammar rules.

You don't have to refer to German grammar since the English grammar in this case contains all necessary ingredients; there is inflection depending on the case:

You don't say "You give I the apple.", but "You give me the apple." (similar for he, she, we, they), i.e. the pronoun is inflected depending on whether it is subject or object, so English speakers are perfectly aware on the difference between subject and object.

When you refer to the subject, you use "who" and when you refer to the object, you use "whom".