How are Celtic languages related to the Indo-European language family?

The Celtic languages constitute a branch of the Indo-European language family. Common Celtic, the reconstructed parent language, shares significant structural similarities with other early Indo-European tongues, such as Latin and Common Germanic, by retaining many ancestral features.12

Key relationships and characteristics include:

• Phonological Evolution: While Celtic is conservative in its vowel system, it exhibits distinct shifts, such as the loss of the Indo-European p and the falling together of certain voiced stops.2
• Classification: Celtic is often categorized within the centum group of Indo-European languages, based on the treatment of specific ancestral palatal sounds.2
• Morphology: The structure of nouns and adjectives remains largely consistent with Indo-European roots. Furthermore, the verb system in Old Irish preserves archaic features—such as the use of primary and secondary endings—that are not found in other recorded Indo-European languages.2
• Italo-Celtic Debate: While some scholars historically proposed a special "Italo-Celtic" unity, this theory has been widely criticized. Shared features, such as passive verb forms ending in -r, have since been identified in other branches like Hittite and Tocharian, suggesting these are not unique to a Celtic-Italic connection.3