Instruction is teaching, and to instruct is to teach. Someone who teaches may be called an instructor.
Ex: Cindy's piano teacher tried with little success to instruct her in the correct fingering for playing Beethoven's Moonlight Sonata. Cindy ended up fumbling around on the keyboard and making a mess of the piece of music.
Ex: Keith's driving instructor fell asleep during an early morning driving lesson, and Keith took advantage of the situation by getting a cup of coffee at the drive-through at McDonald's.
A set of instructions usually accompanies a toy or a piece of furniture that you have to assemble. Instructional describes something that teaches or informs.
Ex: Having to assemble toys for Christmas is every parent's nightmare. The set of instructions is never very clear about which pieces to attach first.
Ex: When Phil needed to replace the base on his blender, he watched an instructional video on YouTube. In no time, he had it put together and was making smoothies for everyone.
A rule is a statement that tells you how to act in a specific situation, such a classroom rule or regulation. To rule is to control, usually a country, and the person who rules is the ruler. A ruler is also a measuring stick, most commonly twelve inches long.
Ex: Mr. Archer has detailed rules for classroom behavior, and, if you break any of them, you will have to serve detention after school.
Ex: Queen Elizabeth II is the ruler of Great Britain, although she has no real power. The position is largely ceremonial today.
As a rule is a phrase that indicates the common or usual way of doing something. You can also say as a rule of thumb. Rule of thumb is commonplace knowledge in a field or the standard way of doing something. The rules of the road include limits on speed and driving on the correct side of the road.
Ex: As a rule, I do my laundry, clean my house, and run my errands at the weekend. Working full time during the week makes it necessary to do those chores on my days off.
Ex: The rules of the road are similar in every state. They include speed limits, wearing seat belts, and using headlights. The most important one is to be aware of other vehicles approaching or following you.