This is my like, 5th one the teacher gave me and I can't think of any more fake/made-up rules that are used in Spanish... Write down what you think is a rule that could explain what makes a sentence grammatically correct or not. For example, you might write something like: "verbs always match nouns in number, and they usually come before the noun." Keep in mind that what you're after is your hunch, NOT a grammar rule from a textbook.

One made up rule I used before was:

"Adjectives come AFTER the noun. For example, chica pelirroja. Chica means girl, and pelirroja means redhead."