Rule – la regla
(Pronounced reh-glah)
To remember that rule is pronounced reh-glah (regal) in Spanish, use the following mnemonic:
They could see that the rule about each colour being refracted by a different amount was true when they aimed the ray at the glass (regla).

Regla is feminine, so it's la regla. Imagine a lady who writes the rules.

Examples of regla (rule) in a sentence
Esta es la regla más importante de la clase.
(This is the most important rule of the class.)
Tienes que seguir la regla si quieres jugar.
(You have to follow the rule if you want to play.)
No entiendo esta regla del juego.
(I don't understand this rule of the game.)