Grammar refers to the rules that we use to generate understandable language. There are rules of plurals and rules of verbs, called word rules. In addition, there are rules of sentence structure, called syntax, discussed in the syntax section. You use these rules every time you write or talk, without even thinking about it. We need these rules to provide structure to our language, so we can understand each other. But we don't need to teach the basics of grammar in school--even kindergarteners know how to speak without any formal training. What are the rules governing words, and how do we acquire them?

Some of the simple rules of linguistics are evident whenever we are asked to create new words. One way that researchers get people to create new words is to show them a strange object and tell them that it is a "tuffle." Then, the researcher shows the subject two of the object, and asks "what do I have now?" The person watching will inevitably reply, "two tuffles." The person has never heard or seen the word "tuffles" before; he just spontaneously generated it, constructing it according to rules he already knew. If I want to create a plural form of a noun, I add "-s"; if I want to create a past form of a verb, I add the suffix "-ed". These endings are examples of function morphemes. A morpheme is a unit that carries some meaning. Whole words, like "tuffle", are called content morphemes, while prefixes, like "anti-", or suffixes, like "-ed", are called function morphemes. The phoneme "ph" would not be a morpheme, because it has no meaning on its own and does not affect a word's meaning when it is added on.

We can learn a lot about the way word rules are constructed by studying how children learn language. In particular, we can study how children deal with irregularities in language when forming rules. For example, the rule for past tense is to add "-ed," but the past tense of "go" is "went," not "goed." Similarly, the rule for plurals is to add "-s," but the plural of "goose" is "geese," not "gooses." When children first begin to speak coherently, around age two, they memorize the past tense and plural forms of all words, which yields the correct form in the case of irregular nouns and verbs. Two-year- olds say, "I went to the store" and "look at all the geese," because that is what they have heard adults say. However, as children become more familiar with the language, they realize that it contains rules that will save them time, so that they don't need to memorize all variations on a word. Three- and four- year-olds will correctly tell you that you have "two tuffles" in response to the above query. However, as they learn these rules, they overapply them, leading to overregulation in their speech. Now, these older children will say "I goed to the store" and "Look at all the gooses," seemingly moving backward in their language learning. Over the next several years, they learn that the rules do not apply to every word, and they memorize the irregular word forms again.

This developmental pattern of language learning is followed universally across cultures and languages. Because it is universal among humans, many researchers have come to believe that there is a genetic component involved in language. Indeed, we seem to be uniquely prepared to learn the words and grammar of language. Other animals can learn to use words in the form of sign language, like chimps, and spoken language, like parrots, but none can employ the complex grammar that human children use already at age three or four. In addition, as discussed in the Biology of Language section, parts of our brains seem to be specially adapted for grammar and word learning. Language itself is not wired into our brain at birth; if it was, we would all speak the same language. Rather, our minds hold the capacity to learn any language that we hear spoken during the first years of life.