It seems to me that you are wrestling with the "Second-Letter Rule",
which says that a single vowel letter (i.e., not combined with another
vowel letter in a digraph) is pronounced tense if the second letter
after it is another vowel letter, lax otherwise. (
lady, caddie, hope, hop, hoping, hopping, June, junk, bite, bit, bitty, bitten, ...)
Unfortunately, this rule has only limited application, typically in
one-syllable words and in final stressed syllables of polysyllabic
words. And the rule never works when
v is involved because
v is virtually never doubled. (
*livved!
*givven!
*devvil!) Several other cases also conspire to make the rule less than universal.
The Second-Letter Rule works for
write and
written, but not for
Donald. You're correct that for the spelling to match the pronunciation in a system where the Second-Letter Rule always works,
Donald would have to be spelled
*Donnald.
I suggest that you introduce a phonetic spelling system to your
students to illustrate how words are pronounced. You may want to
point out some common correspondences between the spelling and the
pronunciation, all the while warning your students that these
correspondences cannot always be relied on, and often the pronunciation
of a word simply must be memorized separately from the spelling.
CJ