In Portuguese there are two accepted reasons vowels in verbs alternate in height.
The vowel harmonizing rule states that where the theme vowel (a from -ar, e from -er, and i from -ir) is deleted from the conjugation and retains tonic stress, the root vowel is changed to match the height of the theme vowel..
For example, the verb
levarhas a root vowel
ewhose height is mid, and a theme vowel
awhose height is low. Therefore, where the theme vowel is omitted from a conjugation, such as in the first person indicative (I lead) the
levowidens from mid to low
The same is true with
e(mid) narrows to
i(narrow), so the first person indicative (I serve) is not
*servo, but it is instead
The second vowel alternation rule is a general lowering of theme vowels for mid height root vowels, where the first rule does not apply.
servirwhose root vowel,
e, is mid is lowered to
ɛwhich is open in many conjugations, especially where the thematic vowel is not changed, and retains the tonic stress. Thus -
sɛrvem. And in
otheme vowel is lowered to
Given that these two are the only accepted vowel alternation conventions in Portuguese, what is causing some speakers to alternate the
cantar to a fronted
æ or even
e? This convention is especially prevalent in the first person form
I'll put a URL to a song here if anybody is interested in hearing it - https://www.youtube.com/watch?v=2WGTENlPlt8#t=31.