Is there any historical grammar explanation why in the tag questions "am" changes into "aren't?"For example, "I'm a teacher, aren't I"? The personal pronoun "I" relates to the first singular person and is used with "am", but in the second part of the sentence there is "aren't" which goes usually with plural forms.Why can't it be "I'm a teacher, am not I"?
It seems to be a remodelling of a form an't, contracted from the awkward amn't.
The OED says (s.v. be, section etymology 2.1):
The forms an’t and ain’t , when functioning as 1st singular, perhaps represent a contraction of am with the negative particle ... an’t and ain’t were subsequently reanalysed as equivalent to are not , and the spelling aren’t superseded an’t in standard English, leading to avoidance not only of I aren’t but also of aren’t I by some speakers of current English.