Which ancient Indo-European features has Lithuanian preserved?

Lithuanian is considered the most archaic living Indo-European language, having preserved numerous features from the ancestral Proto-Indo-European language that have been lost or significantly altered in other related tongues.12

Key archaic features preserved in Lithuanian include:

• Grammatical Number: It retains the use of the dual number in both nouns and verbs in some dialects, a feature present in the ancestral language.13
• Sound Combinations: It has faithfully maintained old sound combinations such as an, en, in, and un, whereas these have evolved into different sounds (such as uo, ie, ī, and ū) in Latvian.3
• Consonant Clusters: It has retained initial clusters like pj and bj, which have undergone changes in other Baltic languages.3
• Palatalization: It has preserved palatalized k and g sounds, which shifted to c (pronounced ts) and dz in Latvian.3
• Locative Case: Old Lithuanian specifically preserved the locative plural ending -su.1

These characteristics make Lithuanian a vital subject for comparative Indo-European linguistics, as its structure often mirrors that of the reconstructed protolanguage more closely than its contemporaries.32