Why does English even have I/me and who/whom? (link.mingpao.com/66886.htm) This confusing language mess is a leftover remnant from the original Old English, often called Anglo Saxon. ('I' or 'Me'? And other leftovers from ancient English) The Angle, Saxon and Jute tribes invaded Britain in the fifth century AD. These tribes all came from the northern German coastal lowlands. They spoke a Low German dialect that became Old English and eventually our Modern English.
In Old English words were sorted by their role in a sentence. Different words and spellings were used depending on whether or not the word was the subject or object of the sentence. These grammatical markers showed exactly who was doing what to whom. In Modern English we use word order to indicate the subject and object of the sentence. So Sally saw Tom. compared to Tom saw Sally. depends entirely on word order to determine who saw whom. Over the centuries English partly gave up cases and switched to using word order, except for the commonly used pronouns. Today only a few very common words, such as I/me, still keep the old system.
To understand that old system one can examine Modern German. Unlike Modern English, German changes nouns, adjectives, articles and pronouns to fit into four grammatical cases. The cases are the nominative, genitive, dative, and accusative. The case of a noun depends on the grammatical function of the noun in the sentence. In English we call the nominative, the subject of the sentence. Example: I am here. The genitive is the possessive. Example: It is mine. The dative is the indirect object. Example: I kicked the football into the goal. The accusative is the direct object of the sentence. I kicked the football.
In that system, the spelling of each noun or pronoun changes depending on the case. Also the articles (a, an, the) change according to both the case and grammatical gender of the word. Latin had two more cases; the ablative for object of a preposition and the vocative for the person or thing addressed. OK, English grammar does not sound as difficult by comparison.
To learn German one must master cases. Chinese languages use word order. To learn English one must master both word order and cases. It is not easy to do.