In English books, I read that the plural of brother was given as brethren, but I hear people in movies or serials using brothers.
Which one is correct?
Is it the difference due to difference in American and British English?
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Use brothers in both speech and writing.
Brethren is a very old plural which is no longer in use, except in very narrow contexts: in works of fiction which depict historical times, or try to create a similar 'atmosphere'; in religious (or quasi-religious) works which embrace the language of the King James Bible; and in works which allude to uses of this sort, either mockingly or affectionately.
The plural of brother as in "male sibling" is always brothers.
He had six brothers and three sisters, so family gatherings tended to be large affairs.
The origin of brethren is an old type of plural for brother, but it is no longer used in that sense. Nowadays, it is only ever used in the context of fraternal or religious organizations: you can be a member of the brethren, but you don't have brethren.
Bottom line: 99.999% of the time, the plural you should use is brothers.
Brothers is the correct term for referring to your male siblings.
In modern English, brethren is uncommon, and it no longer ever refers to your male siblings. It can be used in the following scenarios:
To refer to members of your religious group (esp. some Protestant Christian groups)
The Brethren are meeting on Sunday at 4 for Bible-study class at Julie's house.
Uncommonly it can also refer to some other pseudo-religious or old fashioned clubs and societies:
Meet Jeff. He is one of our Brethren from the Masons' in Bellingham here to visit for a week.
To refer to members of your extended family (uncommon British English, esp. referring to extended family en-masse):
We're going to my grandparent's house this Christmas. The entire Smith brethren are descending on Yorkshire, so we'll also be able to Christine and Jeff.
In fraternal organizations founded back when archaic English was contemporary English (such as the Masons), "brethren" is used to make formal address to assembled brothers, or formal plural reference to brothers. In casual conversation, "brothers" seems to be usual.
There is a corresponding word for a sisterhood - "sistren" - which appears to be having a revival, if Google is any guide.
Those who also know basic Spanish grammar might find it easier to think of word like thee, ye, thou, brethren, and such as being like the formal "Vosotros" form, which have mostly been displaced by the less formal "Tu" form, of you, brother, yes, and such. When teaching English as a Second language, this made sense to the Hispanic kids when helping them with Shakespeare's English.