The minimal units of meaning (morphemes) make up words, and in older Indo-European languages morphology is typologically neither agglutinating (like Turkish) nor analytic (like Chinese and to a lesser extent English), but rather synthetic (fusional).
The oldest Indo-European word forms begin with roots that have canonical root structures, and certain word classes take suffixes and inflectional endings to express grammatical meanings. In addition to suffixes and endings, root vowel ablaut (alternation) may also express grammatical meaning.
Endings are inflectional, and suffixes often have derivational functions, but forms for both inflectional and derivational categories occur with subcategories of word classes, with verbs, adjectives, and nouns or pronouns.
Uninflected (single-morpheme) word classes include particles such as adverbs and sentences connectives.
In the four millennia of attested Indo-European language history, older suffixes and endings are often replaced in later stages by new word classes or new syntactic patterns. Problems that remain in the study of Indo-European morphology include questions concerning the mapping between older IE morphemes, later IE morphemes and syntactic structures, and categories that may be universal or may vary typologically. Innovations that take place over time are also important for issues of dialect subgrouping.