How did Welsh develop from the British Celtic language spoken in Britain?

Welsh is a direct descendant of the ancient British language, a member of the Brythonic branch of the Celtic languages spoken across the island of Britain.123

The evolution from British to modern Welsh involved several significant linguistic shifts:

• Grammatical Simplification: While the ancestral British language was an inflecting language similar to Latin—using word endings to mark noun cases and verb tenses—modern Welsh has largely moved away from such inflectional endings.3
• Phonological Changes: The language underwent extensive changes, including the loss of final syllables and shifts in vowel quantity, which became determined by syllable structure rather than independent vowel length.4
• Latin Influence: During the Roman occupation, the British language was heavily influenced by Latin, which contributed significantly to its vocabulary.4
• Standardization: The language began to be written down around the end of the 8th century, aided by Irish monks who adapted orthography for the vernacular. By the 12th century, a literary tradition emerged, and the language was eventually fixed by the 1588 Bible translation, which helped preserve it despite centuries of pressure from English.45