During infancy, before the use of specific words, children progress from making very simple sounds, including laughter and cooing, to making more playful and experimental sounds, including babbling. The child will then start using protowords, which will often be associated with specific objects. Generally between the age of 10 and 15 months, most children will start uttering their first real words. Protowords and the first use of single words are often referred to as holophrasis. A child can attach complex meanings to these initial utterances, using holophrases to indicate a desire, ask for an object or indicate that something happened.
By 18 months, most children will develop a vocabulary of 50 to 70 words, and then start to use two words at once. Vocabulary learning accelerates, with the baby learning several new words each day. By the age of 2, a child will have a vocabulary of hundreds of words. Nearing the age of 18 months, the child will start speaking in two-word phrases, leading to three- and four-word phrases during the second year. The child begins to form full sentences with subject, verb and object.
During the second and third year, rapid language development occurs. The child's vocabulary grows quickly, along with a more sophisticated understanding of grammar. Children begin to understand the use of verb tenses, prepositions, plurals and possessives. This stage is sometimes called the grammatical morphemes stage, as children experiment and learn markers of proper syntax. Experimentation is evident, as children often over-regularize, using regular morphemes where an irregular word is actually correct, for example saying "catched" instead of "caught," or "foots" instead of "feet."
As the child reaches the fourth and fifth year, language usage becomes developed enough to be considered fluent. The child can speak in full sentences and has an extensive vocabulary. Through the fourth and fifth year, a child becomes able to understand and act upon sentences containing two or more separate parts, such as "Go to your room, get your shoes and bring them here." The child starts using compound and complex sentences by the fifth year.