Because most languages are primarily spoken, an important part of the overall understanding of language involves the study of the sounds of language.

Most sounds in the world’s languages—and all sounds in some languages, such as English—are produced by expelling air from the lungs and modifying the vocal tract between the larynx and the lips. For instance, the sound p requires complete closure of the lips, so that air coming from the lungs builds up pressure in the mouth, giving rise to the characteristic popping sound when the lip closure is released. For the sound s, air from the lungs passes continuously through the mouth, but the tongue is raised sufficiently close to the alveolar ridge (the section of the upper jaw containing the tooth sockets) to cause friction as it partially blocks the air that passes. Sounds also can be produced by means other than expelling air from the lungs, and some languages use these sounds in regular speech. The sound used by English speakers to express annoyance, often spelled tsk or tut, uses air trapped in the space between the front of the tongue, the back of the tongue, and the palate. Such sounds, called clicks, function as regular speech sounds in the Khoisan languages of southwestern Africa and in the Bantu languages of neighboring African peoples (see African Languages).

Phonetics is the field of language study concerned with the physical properties of sounds, and it has three subfields. Articulatory phonetics explores how the human vocal apparatus produces sounds. Acoustic phonetics studies the sound waves produced by the human vocal apparatus. Auditory phonetics examines how speech sounds are perceived by the human ear. Phonology, in contrast, is concerned not with the physical properties of sounds, but rather with how they function in a particular language. The following example illustrates the difference between phonetics and phonology. In the English language, when the sound k (usually spelled c) occurs at the beginning of a word, as in the word cut, it is pronounced with aspiration (a puff of breath). However, when this sound occurs at the end of a word, as in tuck, there is no aspiration. Phonetically, the aspirated k and unaspirated k are different sounds, but in English these different sounds never distinguish one word from another, and English speakers are usually unaware of the phonetic difference until it is pointed out to them. Thus English makes no phonological distinction between the aspirated and unaspirated k. The Hindi language, on the other hand, uses this sound difference to distinguish words such as kal (time), which has an unaspirated k, and khal (skin), in which kh represents the aspirated k. Therefore, in Hindi the distinction between the aspirated and unaspirated k is both phonetic and phonological.

While many people, influenced by writing, tend to think of words as the basic units of grammatical structure, linguists recognize a smaller unit, the morpheme. The word cats, for instance, consists of two elements, or morphemes: cat, the meaning of which can be roughly characterized as “feline animal,” and -s, the meaning of which can be roughly characterized as “more than one.” Antimicrobial, meaning “capable of destroying microorganisms,” can be divided into the morphemes anti- (against), microbe (microorganism), and -ial, a suffix that makes the word an adjective. The study of these smallest grammatical units, and the ways in which they combine into words, is called morphology.

Syntax is the study of how words combine to make sentences. The order of words in sentences varies from language to language. English-language syntax, for instance, generally follows a subject-verb-object order, as in the sentence “The dog (subject) bit (verb) the man (object).” The sentence “The dog the man bit” is not a correct construction in English, and the sentence “The man bit the dog” has a very different meaning. In contrast, Japanese has a basic word order of subject-object-verb, as in “watakushi-wa hon-o kau,” which literally translates to “I book buy.” Hixkaryana, spoken by about 400 people on a tributary of the Amazon River in Brazil, has a basic word order of object-verb-subject. The sentence “toto yahosïye kamara,” which literally translates to “Man grabbed jaguar,” actually means that the jaguar grabbed the man, not that the man grabbed the jaguar.

A general characteristic of language is that words are not directly combined into sentences, but rather into intermediate units, called phrases, which then are combined into sentences. The sentence “The shepherd found the lost sheep” contains at least three phrases: “the shepherd,” “found,” and “the lost sheep.” This hierarchical structure that groups words into phrases, and phrases into sentences, serves an important role in establishing relations within sentences. For instance, the phrases “the shepherd” and “the lost sheep” behave as units, so that when the sentence is rearranged to be in the passive voice, these units stay intact: “The lost sheep was found by the shepherd.”

While the fields of language study mentioned above deal primarily with the form of linguistic elements, semantics is the field of study that deals with the meaning of these elements. A prominent part of semantics deals with the meaning of individual morphemes. Semantics also involves studying the meaning of the constructions that link morphemes to form phrases and sentences. For instance, the sentences “The dog bit the man” and “The man bit the dog” contain exactly the same morphemes, but they have different meanings. This is because the morphemes enter into different constructions in each sentence, reflected in the different word orders of the two sentences.