In contrast to regular verbs, irregular verbs are those verbs that fall outside the standard patterns of conjugation in the languages in which they occur. The idea of an irregular verb is important in second-language acquisition, where the verb paradigms of a foreign language are learned systematically, and exceptions listed and carefully noted. Thus for example a school French textbook may have a section at the back listing the French irregular verbs in tables. Irregular verbs are often the most commonly used verbs in the language. In linguistic analysis, the concept of an irregular verb is most likely to be used in psycholinguistics, and in first-language acquisition studies, where the aim is to establish how the human brain processes its native language. One debate among 20th-century linguists revolved around the question of whether small children learn all verb forms as separate pieces of vocabulary or whether they deduce forms by the application of rules.[1] Since a child can hear a regular verb for the first time and immediately reuse it correctly in a different tense which he or she has never heard, it is clear that the brain does work with rules, but irregular verbs must be processed differently. Historical linguists rarely use the category irregular verb. Since most irregularities can be explained historically, these verbs are only irregular when viewed synchronically, not when seen in their historical context. When languages are being compared informally, one of the few quantitative statistics which are sometimes cited is the number of irregular verbs. These counts are not particularly accurate for a wide variety of reasons, and academic linguists are reluctant to cite them. But it does seem that some languages have a greater tolerance for paradigm irregularity than others. The English verb "pay" sounds regular: "I pay", "I paid", and "I have paid" are all pronounced as expected. But the spelling is irregular and that cannot be perfectly predicted; for example, "pay" and "lay" turn into "paid" and "laid", but "sway" and "stay" turn into "swayed" and "stayed".

Language acquisition is the process by which humans acquire the capacity to perceive and comprehend language, as well as to produce and use words and sentences to communicate. Language acquisition usually refers to first-language acquisition, which studies infants' acquisition of their native language. This is distinguished from second-language acquisition, which deals with the acquisition (in both children and adults) of additional languages. The capacity to successfully use language requires one to acquire a range of tools including phonology, morphology, syntax, semantics, and an extensive vocabulary. Language might be vocalized as speech or manual as in sign. The human language capacity is represented in the brain. Even though the human language capacity is finite, one can say and understand an infinite number of sentences, which is based on a syntactic principle called Recursion. Evidence suggests that every individual has three recursive mechanisms that allow sentences to go indeterminately. These three mechanisms are: relativization, complementation and coordination.[1] The capacity to acquire and use language is a key aspect that distinguishes humans from other beings. Although it is difficult to pin down what aspects of language are uniquely human, there are a few design features that can be found in all known forms of human language, but that are missing from forms of animal communication. For example, many animals are able to communicate with each other by signaling to the things around them, but this kind of communication lacks the arbitrariness of human vernaculars (in that there is nothing about the sound of the word "dog" that would hint at its meaning). Other forms of animal communication may utilize arbitrary sounds, but are unable to combine those sounds in different ways to create completely novel messages that can then be automatically understood by another. Hockett called this design feature of human language "productivity". It is crucial to the understanding of human language acquisition that we are not limited to a finite set of words, but, rather, must be able to understand and utilize a complex system that allows for an infinite number of possible messages. So, while many forms of animal communication exist, they differ from human languages, in that they have a limited range of non-syntactically structured vocabulary tokens that lack cross cultural variation between groups.[2] A major question in understanding language acquisition is how these capacities are picked up by infants from the linguistic input. Input in the linguistic context is defined as "All words, contexts, and other forms of language to which a learner is exposed, relative to acquired proficiency in first or second languages". Nativists find it difficult to believe, considering the hugely complex nature of human languages, and the relatively limited cognitive abilities of an infant, that infants are able to acquire most aspects of language without being explicitly taught. Children, within a few years of birth, understand the grammatical rules of their native language without being explicitly taught, as one learns grammar in school.[3] A range of theories of language acquisition have been proposed in order to explain this apparent problem. These theories, championed by the likes of Noam Chomsky and others, include innatism and Psychological nativism, in which a child is born prepared in some manner with these capacities, as opposed to other theories in which language is simply learned as other cognitive skills, including such mundane motor skills as learning to ride a bike. The conflict between the theories assuming humans are born with syntactic knowledge and those that claim all such knowledge is the product of learning from one's environment is often referred to as the "Nature vs. Nurture" debate. Some think that there are some qualities of language acquisition that the human brain is automatically wired for (a "nature" component) and some that are shaped by the particular language environment in which a person is raised (a "nurture" component). Others, especially evolutionary biologists, strongly object to assuming syntactic knowledge is genetically encoded and provided by automatic wiring of the brain.