The heart of the solfège system is the major scale.
In terms of notes, with the C as the “do”, it will look like this:
C major scale/C ionian:
If we want to sing in a minor key, we have to lower or “flatten” the third, sixth and seventh step. This is done by exchanging the existing vowel with the vowel “e”:
C minor scale/C aeolian:
On the other hand, if we want to raise or “sharpen” the note, we will change the vowel to an “i”. This brings us to the chromatic scale:
C chromatic scale:
In other words, if the note is sharpened we change the vowel to an “i”.
If the note is flattened, the vowel becomes an “e”. The only exception is “re” that when flattened becomes “ra”.