Many people associate syntax and grammar with the rules found in textbooks. How does linguistic research go beyond these written rules? Specifically, how does the work of a research linguist differ from the prescriptive rules used in teaching and language testing? (Questions about Taiwanese; part 4)

First, let's clarify two concepts: descriptive grammar and prescriptive grammar. The languages we know, especially our first languages, have their descriptive grammar already ingrained in our minds. We simply "know how to use" them but can't necessarily "explain" them.

It's similar to how we all know how to walk, but if you ask us to explain the bodily functions involved: how the brain sends commands through nerve signals, how the cerebellum coordinates, how nerve signals travel to the muscles, which muscles contract, and how the semicircular canals in the inner ear coordinate with the brain and muscles to maintain balance while walking—we "know how to do" these things, but we "can't explain" them. To explain them, we need experts in sports science, physiology, and neuroscience to dissect and organize the processes.

The descriptive grammar in our minds works the same way. Let’s take syntax as an example. We use the principles of sentence formation daily, but we can't articulate them. The task of linguists is to use research methods to organize the descriptive grammar in our minds. Why organize it? Why do we want to know how we walk? Why do we want to know about black holes millions of light-years away? The answer lies in science and the pursuit of knowledge: to understand language, to understand our brain's cognitive functions. In practical terms, it can be applied to language teaching.

In other words, descriptive grammar is the grammar of the languages we naturally know how to speak; we "don't know" it consciously, nor do we particularly want to know it. 

Contrary to descriptive grammar, what we generally find "difficult" is prescriptive grammar. We only need to learn prescriptive grammar when we study languages we don't know, like when I learn English as a second or third language. 

Prescriptive grammar is a standardized grammar developed for "teaching" and "testing." Because the grammar of a language varies by location, age, and individual, a standard is needed for teaching and testing. For example, when we go to English-speaking countries like the United States or watch English movies, we often hear people say, "I didn’t do nothing." This type of double negation is a usage that we absolutely cannot use in tests like TOEFL, TOEIC, or IELTS; otherwise, we will lose points because it doesn't conform to the "prescriptive grammar" of English teaching and testing. Yet, this is precisely a grammatical form that some native English speakers use, which is "descriptive grammar."

In sum, linguists work to find the descriptive grammar of languages. These findings are then standardized to make the prescriptive grammar.