Cúmhaí wrote:
From what I have heard, in addition to "go mb'fhearr" for "gurbh fhearr" you will also hear "an mb'fhearr" instead of "arbh fhearr"
I have not, however, heard "Is b'fhearr liom" so I think this is just for dependent forms (if that is the correct term

)
"
Is" is a copula form (present tense), "
b'" is a copula form (past/conditional). So "Is b'" doesn't make any sense.
In "
an mb'fhearr" "
an" is a verbal particle. The copula
ba follows (shortened to b' and eclipsed, so
mb')
an mb'fhearr = an + ba + fearr
In "
arbh" three different words are combined:
an (interrogative verbal particle),
ro (perfect verbal particle) and
ba (past copula)
arbh fhearr = an + ro + ba + fearr
So, the difference between "arbh fhearr" and "an mb'fhearr" is the occurence or absence of the perfect particle
ro, shortened to the letter
-r-.
AFAIR,
ro occured originally only in past tense but
not in conditional mood (as it still does with most "normal" verbs, eg. an mbainfeadh sé? vs. ar bhain sé? ar = an + ro)
So the difference between
arbh fhearr and
an mb'fhearr (and between
gurbh fhearr and
go mb'fhearr) was the difference between past tense and conditional.
But this difference is blurred and in dialects both forms compete in all historical tenses (past, imperfect, conditional). Forms with ro prevail and made their way into Standard Irish.
(though even in Standard Irish two phrases without ro,
go mb'fhéidir and
go mb'fhiú, should be used acc. to GGBC).