The utterances children hear don't consist of bare words strung together. Rather, depending on the language, words are usually modulated to include further information about the specific meaning to be conveyed. These modulations take the form of inflections, usually suffixes, added to word-stems, and of freestanding grammatical forms like prepositions. Some languages indicate the roles played by the referents of each noun phrase (e.g., agent, recipient, place, instrument) through word-endings added to the noun. They mark the doer of the action with nominative case, or where the event took place with locative case. On verbs, they can mark when an action took place with tense marking on the verb, or the general temporal “shape” of an action – whether it was completed, reiterated, or lasted for some time – with an aspectual ending on the verb, and so on. They may also mark gender on nouns (e.g., masculine, feminine, neuter) as well as on articles, adjectives, and sometimes verbs; and they can mark person (e.g., first, second, or third person on the verb) and number (e.g., singular or plural on nouns, verbs, and adjectives).

Modulations like these are generally provided by the inflections of a language, but languages differ in how they add such information to nouns and verbs, the regularity of the forms they use, and the division of labor between grammatical particles (inflections or free grammatical morphemes) versus reliance on word order.