It is to 'ail'
If that could be "i," the answer is "ill."
Ill
ail example: What ails you?
I've is a word that means I have. I'll is a word that means I will.
Ick, ice, ire, ink, ill, ilk,
Bee, ill, all, tee, odd, add.
A 3-letter word that means "which person" is "who."
and
but
Ado.
Let
sag