Grammar is a collection of rules that make up a language. Grammar is also used as a title to refer to the usual laws of a given language, which may change over time. The way people spoke in the 1700 ́s is not the way we talk today.
Grammar may be separated into two common categories: descriptive and prescriptive. Both views of grammar are used today, although linguists usually lean towards a descriptive approach to grammar, while people teaching a specific language (English) might tend towards a more prescriptive approach.
Descriptive grammar
Descriptive grammar attempts to look at the grammar of any language as it is used. It is determined by the rules of the speech group that it is spoken in rather than the set of laws deciding whether a sentence is grammatical or not. For example, in various speech societies, a sentence such as “Ya ́ll ain’t getting’ that” would be entirely grammatical, and a whole set of grammar rules can be deduced to explain why that formation is used. In another speech community, however, that sentence might be deemed ungrammatical, and the only accepted version, such as, “You are not getting that,” would be viewed as acceptable.
Prescriptive grammar
Prescriptive grammar covers the norms of speech as given by authoritative sources. It creates rigid rules by which all speech within that language must obey to be deemed grammatical. Fewer linguists use a prescriptive approach to grammar in current times, favouring to describe language as it exists in a given speech community. However, many teachers and pedagogues still use a prescriptive approach towards grammar, adhering to fixed rules as the only correct way to speak.
Prescriptive grammar is also used to an extent in teaching non-native speakers a language. For example, it can be useful to use a standard form of English as a baseline to train from when teaching English. This typically helps to reduce confusion with students. Once the language has been achieved, of course, a less-prescriptive approach will surely gain control, as the non-native speaker studies regional rules and language that might not fit the prescriptive grammar he or she first learned.