Grammatical morphemes are used to modulate word meanings and to link words in constructions. They consist of inflections, usually suffixes, added to words, and free-standing function words (prepositions and articles). Different language-types make different uses of these, including cases added to each noun, tense and aspect markers added to each verb, and agreement markers linking nouns, adjectives, and determiners. Children have to identify each inflection, its meaning, and where it is used on each word class. They start to add modulations to their words as soon as they start to combine words (and may understand some of them before this). They use regular forms as their starting point and over-regularize irregular forms. They show some consistency in the order of acquisition for different modulations, depending on the semantic complexity of each grammatical morpheme. Semantic complexity, formal complexity, and frequency all play a role here. Children may initially rely on filler-syllables, and only later produce the relevant form. Word class plays a role here, since the choice of grammatical morphemes depends on this. Initial use of grammatical morphemes may be limited to specific words and only later extended. The same holds for agreement in gender and number.