- A principle or order which guides behavior, says how things are to be done or
- The usual way that something happens.
With regard to grammar, the first type of rule is often called a prescriptive rule and the second a descriptive rule. For many people, grammar instruction is traditionally associated with the teaching of the first type of rules – that is, prescriptions as to what should be said or written: “Do not use different to and never use different than. Always use different from.” “ Never use the passive when you can use the active.” “Use shall for the first person and will for second and third person.”
Second and foreign language teaching, on the other hand, is primarily concerned with descriptive rules, that is, with generalizations about what speakers of the language actually do say rather than with what they should do. Thus: “You do not normally use the with proper nouns referring to people”. “We use used to with the infinitive used to do/ used to smoke to say that something regularly happened in the past but no longer happens.” 
Until recently most so-called descriptive rules were based on hunches and intuitions. There is much greater authority in descriptions of language since the advent of large computer databases of naturally occurring language known as corpora. The following rule, for example, represents the traditional wisdom with regard to some and any:
1. “As a general rule, use some in affirmative sentences, use any in questions and negative statements.” 
Statistical evidence provided by corpora has indicated that this rule oversimplifies the issue and that the following qualification needs to be made:
2. “Any can mean it does not matter which. With this meaning, any is common in affirmative sentences.”.
This brings us to a further distinction that needs to be made with regard to descriptive rules. Compare, for example, rule 1 with the following:
3. “The primary difference between some and any … is that some is specific, though unspecified, while any is nonspecific. That is, some implies an amount or number that is known to the speaker. This difference tends to correlate with the difference between positive and negative contexts.” 
Rule 3 may be the truth, but most learners of English (and many teachers) would find such concepts as specific, nonspecific, and unspecified difficult to untangle. Rule 1, on the other hand, makes up in simplicity for what it lacks in truth. It is accessible to learners and, as a rule of thumb, it will probably serve quite well until such time as the learner is ready to tackle a more truthful rule, such as rule 2. We need, therefore, to define a third category of rule: pedagogic rules – rules that make sense to learners while at the same time providing them with the means and confidence to generate language with a reasonable chance of success. Inevitably, such confidence is often achieved at the expense of the full picture. Teachers must, in the end, cater for the learner’s needs rather than those of the grammarian.
With regard to pedagogic rules, a further distinction may be made between rules of formand rules of use. The following is a rule of form: “To form the past simple of regular verbs, add –ed to the infinitive.” 
This, on the other hand, is a rule of use: “The simple past tense is used to indicate past actions or states.” 
Rules of form are generally easier to formulate and are less controversial than rules of use. It is relatively easy to explain exceptions, such as carried, loved, stopped to the above rule of form for the past simple and to construct fairly watertight sub-rules that will handle them. But the following exceptions to the rule of use are less easily accommodated into a general rule about the past simple:
How did you say you spelt your name; I was wandering if you had any detective novels.
Rules of use, being heavily dependent on contextual factors, are seldom captured in terms that are black or white. The slippery nature of rules of use can be a cause of frustration for both learners and teachers alike, and is one argument that supports the teaching of language through examples or by means of contexts.