English gender is the odd one out in its own family
Most languages related to English sort nouns into grammatical genders, and speakers must memorize which noun takes which. English does not work that way, and that makes its gender system unusual within the Indo-Germanic family [1]. The striking part is that what looks like a "natural" system to English speakers — he for men, she for women, it for everything else — is not the family norm. It is the exception. So the weird rule may be the one we never notice: English assigns gender largely by meaning rather than by arbitrary noun class [1].
Why can a mouse be a "who"?
The relative pronoun "who" is supposed to be for people, with "which" or "that" for everything else. But usage does not stay in that lane. A study of the pronoun with nonhuman animals found that some language specialists accept "who" for animals, and that real spoken and written English contains such usage [5]. The paper frames these as minor exceptions to the general rule rather than a wholesale change [5].
That matters for how you read the rule. The "people only" version is a strong tendency, not an absolute law, and the boundary is fuzzy enough that careful writers disagree about where it sits [5]. If you have ever written "the dog who lives next door" and then second-guessed yourself, the evidence suggests you were not simply wrong — you were using a recognized exception [5].
Some of the weirdest rules were never rules
A different kind of oddity is the rule that exists only because someone said so. Work on English grammar education describes persistent misconceptions and "ghost rules" — prescriptions that circulate widely but do not reflect how English actually works, and that even scholars of English literature often hold mistakenly [3]. These are weird not because the language is strange but because the rule is a piece of folklore wearing a grammar book's clothes [3].
Negation offers a related puzzle. English and its relatives have repeatedly renewed their negative markers over history, a cycle in which a word like "not" gets reinforced and then replaced — a pattern later named Jespersen's Cycle [2]. The same comparative work documents double negation and language-specific variations in how negatives are expressed [2]. So a construction that sounds like an error in one dialect can be a normal stage in a pattern that recurs across languages [2].
The weirdest rule may be the one you cannot state
Rules and routines in language production are not the same thing: syntactic rules generate sentences, while idiomatic routines are stored chunks a speaker retrieves whole [4]. That split helps explain why some English "rules" feel unstatable — you can use them fluently and still fail to explain them. The oddest rules may be the ones that live in routine rather than in any grammar book [4].
Put together, the evidence points to a simple answer: English's weirdest rules are the ones where the official story and the actual usage diverge — a gender system unlike its relatives [1], a pronoun that slips its leash [5], ghost rules that were never real [3], and negation patterns that keep rebuilding themselves [2]. The language is less rule-bound, and more historically restless, than the schoolroom version suggests.
Sources used in this answer
Gender shifts in the history of English
English's gender system is unusual compared with other Indo-Germanic languages, making it an atypical feature within its own family.
Negation in English and other languages
Negation across languages shows recurring patterns including double negation and the cyclical renewal of negative markers later called Jespersen's Cycle.
The unfortunate divorce of English grammar from English literature
English grammar education carries persistent misconceptions and 'ghost rules,' and scholars of English literature often hold mistaken beliefs about English grammar.
Rules and routines: A consideration of their role in the task-based language production of native and non-native speakers
Language production draws on both syntactic rules and stored idiomatic routines, which are distinct kinds of knowledge.
Elephants who marry mice are very unusual: The use of the relative pronoun who with nonhuman animals
The relative pronoun 'who' is sometimes accepted and used with nonhuman animals, treated as a minor exception to the general rule.