Once upon a time, grammar was rules - mainly rules about what not to do, such as not splitting infinitives or doubling negatives. More recently, the slogan has been "Tools, not rules" - a very healthy change, we think.
This puts the emphasis on the positive side of grammar, and it's why we called this column "The writer's toolkit". Every grammatical pattern is a tool that helps a writer (or speaker), and successful adult writers probably have thousands of grammatical tools in their toolkit.
Having said all of that, we want to come back to rules because, however much we appreciate the toolrule rhyme, we think the contrast is misleading. The point is that every grammatical tool works because it is a rule - a regular relation between a "meaning" and a "wording".
For example: adding - ed to a verb shifts the meaning into the past; adding "if" to a clause turns it into a condition; a colon means that what follows is an expansion of what went before.
These are rules, but, unlike the rules of the bad old days, they tell us what to do, not what not to do. In technical jargon, these rules are descriptive not prescriptive; they describe normal usage, rather than trying to change it.
These rules work in just the same way as the rules of the Highway Code.
"Drive on the left" is a very handy tool for preventing accidents. If you drive on the left, you'll be safe because everyone else will be doing the same and they'll understand you. Similarly, if you use - ed to move a verb's meaning into the past, you'll be understood, and you can understand others when they do the same. The rules are tools for helping us to achieve our aims.
A good way to explain these basic ideas to a class is to take them through the rules for using grammatical subjects and objects. Each verb describes a little scene in which different characters play different roles, so it's important to show (and understand) who's who. Rule 1 says: "Put the subject before the verb and the object after it", and then Rule 2, which varies from verb to verb, says which character is the subject and which is the object. For example, "Jane likes Ahmed" means that Jane, not Ahmed, has the feelings.
This isn't something you need to teach key stage 3 pupils, but they should learn not to take their rules for granted - they're not fixed for all time.
In fact, Rule 2 for "to like" has changed since Shakespeare and Pepys, who wrote "the houses did not like us". You can guess what he meant in that sentence, but what if he had written "They like us"? Without the right tools, understanding breaks down.