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'.
singular of the word stitches is stitch.
Steroids is plural. The singular form of the word is steroid.
Singular
the word 'know' is neither singular nor plural. This word is a 'verb' and is in the present tense.
The word crisis is singular. The plural form of crisis is crises.
The singular form of brethren is "brother."
Brethren is plural. It means "brothers".The singular form would be "brother".
The Ilocano word for brethren is "kakailian."
In the King James version the word - brethren - appears 563 times the word - brethren's - appears once
The Greek word for brethren is "ἀδελφοί" (adelphoi).
singular
Bacterium is the singular word.
the singular word it is salmon.
The singular word for adenomata is adenoma.
The word 'Phenomenon' is singular. The plural form is 'Phenomena'.
Lance is singular.
Yes, the word "document" is singular.