What is the singular word for brethren?

Answer 1

The word 'brethren' is an archaic (historical) plural of our word 'brother'. According to www.etymonline.com, this older form of the plural was commonly used between the 13th and 17th centuries, before the modern form 'brothers' began to be used.

'Brethren' is still an English word, but tends to be reserved for religious usage (or intentionally archaic uses).

The -en ending for plurals in English is a Middle English form (the form of our language predominant between about the 11th and 15th centuries) based on an Old English (the oldest form of English, spoken 5th-11th centuries) weak declension - for this reason you'll never see a native word in common usage which has obviously been introduced after that time pluralised with -en ('computeren', for instance, or 'helicopteren' - although there is an example in 'Leet-speak' of a slang term for computer, 'box', pluralised to 'boxen', this is a humorous usage).

Other examples of words which use the -en plural form are 'children' and 'oxen'.