"Betwixt" is archaic and highly marked for American English, but not technically wrong.
"Learnt" is non-standard, but intelligible and probably not a problem.
I've never heard or seen the word "trebble", and would mark it as an error in any piece of formal writing.
english.stackexchange.com/users/48/JSBձոգչ
"Treble" (in British English at least) can mean the same as "triple", as well as high in (musical) pitch, as in the opposite of "bass" (see also the Wiktionary entry ).
"Learnt" (again, British English) is an alternative past form of "learned" ( Wiktionary again ).
"Betwixt" is a great word, but is somewhat archaic ( Wiktionary, one more time ).
Despite that, either "twixt" or "betwixt" was used in the film Serenity .
english.stackexchange.com/users/303/Steve Melnikoff
My $.02 USD:
Betwixt: not commonly used nationwide
; I've heard it (and used it) in New England.
Trebble: We don't double up the b.
It's spelled 'treble' and it is still in use.
In American English, we don't substitute the -ed ending on verbs with a t. Learnt
= learnedSpelt = spelled and so on. Without wanting to speak ill of Texas...
it's Texas.
They have their own dialect of English down there.
As a bit of anecdotal evidence, my nickname is "Lin" (short for Linda).
My Texas friends manage to take the letters Lin and somehow stretch two syllables out of them. "
Le-in" it becomes.
Why?
I have NO idea.
english.stackexchange.com/users/9479/Darwy
Treble might be more acceptable if the US played darts more often.
In BE, the 3x multiplier ring of a dart board is called the treble ring of which the highest scoring segment is the treble twenty. http://www.pdc.tv/staticFiles/b6/b3/0,,10180~177078,00.pdf
english.stackexchange.com/users/1778/ukayer
I've never heard an American use betwixt, trebble (or aught, naught, or nought), or learnt, although: "betwixt and between" as a figure of speech would be "acceptable".
"larnt" is an Appalachian dialect word that would cost you points in a school paper.
english.stackexchange.com/users/152/Taldaugion
I'm American, and the only place I've ever seen treble used was on computer speakers and car stereos (as in bass and treble ).
english.stackexchange.com/users/34/Gary
I grew up speaking British English. The words I learnt were occasionally marked off in papers, despite their being English words. Are words like  betwixt ,  trebble ,  learnt  acceptable in papers for English classes for professors in America, specifically Texas?
Are "betwixt", "trebble", etc., acceptable in American English?
english.stackexchange.com
<american-english><british-english><formality>
english.stackexchange.com/questions/21
english.stackexchange.com/users/33/EricR
It is generally considered that "betwixt" is an archaic word but not technically wrong. "Learnt", however, is non-standard in American English. "Trebble" does not exist, unless the intended word is "treble". Some of these terms may be technically correct but they are still unusual so best avoided in formal documents.
It is generally considered that "betwixt" is an archaic word but not technically wrong. "Learnt", however, is non-standard in American English. "Trebble" does not exist, unless the intended word is "treble". "Betwixt" is archaic and not commonly used in American English, but not technically wrong.   "Learnt" is non-standard, but intelligible and probably not a problem.  "Treble" in British English at least can mean the same as "triple", as well as used in music. In American English it is rarely used, except for speakers.
"Betwixt" is archaic and not common in American English, but not technically wrong. Similarly, "Learnt" is non-standard, but intelligible.  However, "trebble" would be marked as an error in formal writing, in favor of the current "treble". Whilst "betwixt" and "learnt" are considered either non-standard or archaic, they are generally not considered a problem in an American academic setting. There is less agreement about the word "trebble"/"treble".
Betwixt' is archaic but not wrong. These are unusual in American English. Betwixt' is used in the US but only in certain areas like New England, and 'trebble' is used widely with one 'b'. None of these are used in US English commonly except for the phrase 'betwixt and between'. Treble' is only used for computer speakers and car stereos.
"Betwixt" is archaic and not commonly used in American English, but not technically wrong.   "Learnt" is non-standard, but intelligible and probably not a problem.   "Treble" in British English at least can mean the same as "triple", as well as used in music. In American English it is rarely used, except for speakers.
Whilst non-standard or archaic, Betwixt and Learnt are not considered a problem in American English classes. There is some disagreement as to whether the word "treble/trebble" is acceptable.