Without rules language would be unable to convey meanings effectively. For example, there's a huge difference in meaning between The dog bit the man and the The man bit the dog. In English the only thing that tells us who is doing the biting and who gets bitten is the word order (subject - verb - object), unless the verb is passive.
There's also a difference in meaning I don't think so and I would hardly have thought so. Again, this is governed by a rule(s).
The formation of the plural of nouns, of the tenses of verb, the order of adjectives and word order, for example, are rule-governed. These aren't rules worked out by nasty people who want to trip you; they are regularly occurring features of English.